IPD (I.P.D.) is the acronym used to describe a particular type of Reyvateil unique to the region of Metafalss, their defining characteristic being that they are not connected either to the First Tower of Ar tonelico in Sol Ciel or the Tower of Origin in Sol Cluster. Instead, they use as their Song Server a device created especially for them called Infel Phira. The acronym that gives them their name stands for Infel Phira Dependent.
The origin of the IPD Reyvateils can be traced back to the earnest and desperate wishes the Metafalssians had for creating their legendary utopia, Metafalica. While the first attempt at making a method for Metafalica's creation had already been done in year 3080 AD with the creation of the Hymmnos EXEC_METAFALICA/. by Jaza, she sealed it away and kept it hidden because said Song was impossible to sing for any living Reyvateil and would only cause the death of the singer in exchange for creating a flawed land.
Therefore, the Grand Bell, the ruling organization of Metafalss dedicated themselves to investigating a way for making Metafalica a reality and to decode Jaza's EXEC_METAFALICA/. The latter approach hit a dead end as soon as the discovery of EXEC_METAFALICA/. being impossible to sing was made, which caused the people a great depression after the Grand Bell had boasted that decoding Jaza's theories would lead them to eternal prosperity. However, the first approach: the creation of Infel Phira finally saw fruits after multiple failed attempts when Professor Grammul appeared in scene in year 3258 AD.
Grammul wrote a thesis based on a Super Reyvateil Theory he had made himself, which stated ideas such as creating an independent Song Server to not have to rely in the unfathomable powers of Ar tonelico and making Reyvateils customized specifically for this new Server. Grammul also demonstrated in year 3274 AD that Infel Phira could in fact be possible, which got him the green light from the Grand Bell to start working on its construction the following year.
After the basis was complete, Grammul and his team started working on Infel Phira's First Stage in year 3285 AD, with this stage being completed five years later. Grammul then started experimenting on Reyvateils by modifying their Spectrum Genes in order to switch them over from Ar tonelico to Infel Phira, but all these attempts ended in failure with the girls dying from the destruction of their minds. Finally, he managed to successfully transcribe an orphan girl to Infel Phira in 3291 AD, thus giving birth to the first IPD: Infel.
Grammul ultimately named and adopted this same girl as his daughter after the experiments when he realized how intelligent she was, despite the fact that under his current theories, Infel herself would have to be sacrificed for Metafalica's creation. On the meantime, Grammul also started carrying out Spectrum rewrites on several other Reyvateils in order to have the IPD numbers that would be necessary for Metafalica. However, ultimately Infel proved to be better than him as a researcher, as she discovered several flaws on Grammul's theories when she studied them in 3304 AD and even proposed a better method to create Metafalica based on the Hearts of the Land she had discovered that would need no sacrifices to be carried out, which caused Grammul to be fired from the Grand Bell and his position in the project to be given to Infel instead.
In 3308 AD, Infel made and programmed the Hymmnos dialect exclusive to the IPDs: the New Testament of Pastalie, helped by the crew of engineers and scientists that she inherited from Grammul, which had been renamed to People of Mio. This team then embarked on the Second Stage of the Infel Phira project, and during its course Infel and Nenesha sang in 3310 AD together a prototype version of Metafalica to ascertain it was working properly. This experiment was successful and it produced a small Heart of the Land that the team dubbed Metafalica-α in 3310 AD, thus leading the Second Stage to be declared as successfully completed.
However, the real attempt at creating Metafalica in 3313 AD was a complete failure due to the fact that the IPDs didn't follow Infel's lead and instead, they refused to sing with her once they saw her true feelings, causing Nenesha's death and the start of the First War with the Goddess. Infel Phira sustained a great deal of damage during the course of this war, causing the death of many IPDs and at same time destroying the failsafes it had for their Soulspaces. This last consequence led into the recurring problem of the IPD Outbreaks, in which IPDs go berserk and destroy everything around themselves with Song Magic, which together with the treaty the Grand Bell signed alongside the Goddess' representatives in 3316 AD, prompted them to hunt down the IPDs as they had their outbreaks, even though they had a tremendous population increase in 3320 AD. Things got softer with the discovery in 3348 AD that Dive Therapy could be used to quell the outbreaks, to the point the Grand Bell ordered the profession to be unbanned and to initiate investigations on the relationship between it and the IPDs the next year, even though the preferred approach was slaughtering them to reduce their numbers so not many would remain when the time for executing the Hibernation Project came. 
These policies toward the IPDs only changed in 3753 AD, when Chancellor Alfman staged a coup d'etat in which the last Pope was killed, and he usurped his seat as the most powerful man in Metafalss. He then established a Chamber for Investigating and Quarantining IPDs the next year in order to deal with the problem of the outbreaks, but it didn't change much: whenever an IPD had an outbreak, she would be detected using the Lakra System and a troop of Grand Bell Knights would be sent to contain them, even though killing them in the process was also a typical result. Alfman also banned the practice of Dive Therapy in the capital city of Pastalia and planted the notion it was indecent so no one outside the Grand Bell could treat the IPDs, while he also maneuvered in the shadows to increase their numbers, both of these actions being to build an army for their War Against the Goddess, causing the IPDs to still be regarded with fear and pity by the Metafalssians at large.
The situation finally took a turn for the better when Cloche Leythal Pastalie assumed the power as the Maiden after Alfman fled Metafalss and Targana's attempt at carrying out the Hibernation Project failed. These events led her to sing EXEC_with.METHOD_METAFALICA/. successfully alongside Luca Trulywaath, finally creating the land of Metafalica and fixing Infel Phira's internal status so the IPD Outbreaks wouldn't trigger ever again. As the IPDs are tasked with the maintenance and preservation of the continent, they are now treated with respect like all other Reyvateils in Metafalss, and it is likely that due to their current role, they will become even more revered as time goes on.
Unlike the other Reyvateil types, the IPDs aren't created from zero but are instead modified versions of the Third Generations that are born as hybrids of Pureblood Reyvateils and humans. Therefore, they suffer from their same weakness: an extremely short life span from Infel Phira devouring their life force that doesn't go beyond 20 years and that requires a trimestral dose of Diquility to be counteracted.
However, what makes IPDs special is the fact that they use the New Testament of Pastalie for their Songs, which thanks to its compressed grammar and vocabulary, its words being optimized to contain more power and Infel Phira's reduced instruction count, it allows them to sing faster than other Reyvateil types and with greater amounts of power. And while they get the best effect from singing in New Testament of Pastalie, the IPDs are also capable of using all other Hymmnos dialects excepting for the Cluster and Alpha Notes due to Infel Phira emulating them, though doing so would reduce the overall effect the Song would ultimately have and its execution speed.
Additionally, while the IPDs do have the Hymn Code zone in their Soulspaces, it goes unused: Infel Phira authenticates the singers for its Hymmnos Extracts depending on their Potential Level, or IPD Level, and on whether they possess the Queen Registry or not. This makes it possible for all IPDs to Download the Hymn Crystals for Infel Phira while also restricting the access to particularly powerful or important Songs only to those that are suitable to sing them.
The other remarkable difference is the fact that IPDs only have their Individual Consciousness Fields, or Cosmospheres, down to the frequency range where Level 7 is located: the other two levels were instead repurposed as part of the Shared Consciousness Fields and are called E-Spots. Therefore, no Diver is capable of Diving into these levels and thus the Completion Ceremony is conducted at Level 7, though it's possible to unlock them for Diving by turning a special lock called the Boundary Key, which exists in Level 7, though this requires the Maiden of Mio to have sung METHOD_METAFALICA/. first.
This also means that the IPDs don't have Mind Guardians, as given the lower depth at which their Cosmospheres end and Infel Phira's own specifications, they are unable to craft powerful individual feelings on the same way that the Ar tonelico and Harvestasha dependents can. On the other hand though, the existence of the E-Spots makes it possible for them all to share their feelings with each other, which can be utilized on particularly powerful Songs. In fact, the E-Spots themselves are instrumental to the execution of the Songs METHOD_METAFALICA/. and METHOD_REPLEKIA/., as well as the sealed modes of Messela, Torcarrol and Deenelmaite depending on them for their functions.
However, this same sharing of emotions is a double-edged sword due to the fact that while all IPDs can achieve truly impossible feats when they have their feelings in harmony, when there is discord between them, it is extremely easy for negative feelings to spread among them, causing the E-Spots to fail to carry out the function they were designed for. And in fact, the E-Spots together with an abuse of the Torcarrol Mode are the underlying cause behind the IPD Outbreaks.
The IPDs are the least numerous Reyvateil type in Ar Ciel due to existing solely in Metafalss. And even while Metafalss has the highest Reyvateil-to-human ratio proportion in the world, not counting Clustania; of the approximately 339600 Reyvateils who live upon this region only about a 48% are IPDs, with a fifth of them being in containment by the time Ar tonelico II: Melody of Metafalica begins.
- No equivalent to the Pureblooded β-Types or Origins exists for Infel Phira. And while it is theoretically possible to create β-IPDs, it would be extremely complicated because the theory behind the original βs' creation has been lost to time, so their design would have to be made from zero. Infel also mused once that it would be possible to create them by just transcripting an Ar tonelico or Harvestasha β to Infel Phira, but no one was interested in attempting it.
- While there was a certain degree of confusion among some players about the exact way in which a Reyvateil becomes an IPD, it has been made clear that IPDs are so by birth, and not by any other reason. The reason why it is considered a genetic disease by the time Ar tonelico II: Melody of Metafalica takes place is a combination of the Negative type outbreak with the normal Reyvateil awakening symptoms, but it has nothing to do with diseases or viruses. Therefore, all other Reyvateil types can never be affected by IPD outbreaks nor can they become IPDs.
- ↑ 1.0 1.1 Ar tonelico II: Melody of Metafalica Setting Encyclopedia - History Section, Page 91
- ↑ 2.0 2.1 2.2 2.3 2.4 Ar tonelico II: Melody of Metafalica Setting Encyclopedia - History Section, Page 92
- ↑ 3.0 3.1 Ar tonelico II: Melody of Metafalica Setting Encyclopedia - History Section, Page 93
- ↑ Ar tonelico II: Melody of Metafalica Setting Encyclopedia - History Section, Page 96
- ↑ Ar tonelico II: Melody of Metafalica Setting Encyclopedia - History Section, Page 110
- ↑ Ar tonelico II: Melody of Metafalica Setting Encyclopedia - History Section, Page 99
- ↑ 7.0 7.1 Ar tonelico II: Melody of Metafalica Setting Encyclopedia - World Section, Page 106
- ↑ Ar tonelico Qoga: Knell of Ar Ciel Drama CD Vol. 3 -Side Tyria After Story- - Track 6
- ↑ Ar tonelico II: Melody of Metafalica Setting Encyclopedia - History Section, Page 94
- ↑ Ar tonelico II: Melody of Metafalica Setting Encyclopedia - World Section, Page 104
- ↑ Ar Portal - Toukousphere, Issue #63 - Technical Service Center
- ↑ Ar Portal - Toukousphere, Issue #80 - Technical Service Center
- ↑ Ar tonelico II: Melody of Metafalica Setting Encyclopedia - History Section, Page 94
- ↑ Ar tonelico II: Melody of Metafalica Setting Encyclopedia - History Section, Page 97
- ↑ Ar tonelico II: Melody of Metafalica Setting Encyclopedia - World Section, Page 107
- ↑ Ar Portal - Toukousphere, Issue #119 - Technical Service Center
- ↑ Ar Portal - Toukousphere, Issue #47 - Technical Service Center