DiMA, alongside Nick Valentine, is one of two early prototypes created by the Institute to test if synths could handle independent thinking and judgement. While DiMA was allowed to freely develop his own personality through experience, Nick was outfitted with alternating, pre-existing human personalities. The two formed a close sibling relationship while in the Institute together, in DiMA's words: "I literally saw myself in you". DiMA, not being able to bear seeing his brother put through such cruel experiments, decided to run away with Nick and the duo eventually managed to escape. However, disoriented by the experiments done to him, Nick did not recognize DiMA after escaping. Believing himself to be the human Nick Valentine, Nick attacked DiMA, forcing DiMA to knock him out and then abandon him.
Afterwards, DiMA traveled to Far Harbor to hide from the Institute, though without a programmed objective, sat in a cave doing nothing for nearly a year before it occurred to him that he could define his own path in life. He then went on to live in an old nuclear-armed submarine base that was eventually discovered by the Children of Atom. DiMA befriended their leader, Confessor Martin, and eventually gave them the base as their home.
Alongside Faraday, a synth scientist that acts as his assistant, and Chase, an ex-courser that DiMA was able to convince to defect from the Institute, DiMA went on to found Acadia with the intention of creating a safe place for synths to live freely and not have to hide their nature. From there, DiMA was able to develop fog condensers to protect the residents of Far Harbor from the Fog, which consequently angered the Children, as the Fog is sacred to them.
With tensions high between the Children, Acadia, and Far Harbor, DiMA, desperate for peace and safety for his own people, created contingency plans in the event that either group endangered Acadia--for the Children, it was acquiring the key to the nuclear missile in the Nucleus, and for Far Harbor it was acquiring the shut down codes for the town's wind farm and only power source. Then, in his desperation, DiMA chose to kill Captain Avery and secretly replace her with a synth as to have a "moderate voice" to be an example of how synths and humans could live side by side. DiMA was so sickened by his own actions that he chose to remove these memories and, as a sign of good faith, put them in the Nucleus' computers for the Children to safeguard.
Sometime later, DiMA and Nick are reunited when Nick and the Sole Survivor travel to Far Harbor looking for Kasumi Nakano. While DiMA expresses surprise and excitement to see Nick again, Nick is far more reluctant, not having any memories of DiMA due to their last encounter being over a hundred years prior--memories DiMA has been able to maintain through his many modifications. However, if the Sole Survivor encourages Nick to accept DiMA as his brother after finding the holotape of their fight, Nick will apologize to DiMA and the two will occasionally greet each other or chat when entering Acadia.
Based on the player character's actions, DiMA can become an ally or an enemy.
If the player character opts to keep DiMA's secret, he will assist in replacing High Confessor Tektus with an obedient synth, in order to secure a peace between the Children of Atom and Far Harbor.
If the player character successfully convinces him to turn himself in, he will go to Far Harbor, confess what he did and then be executed by the townsfolk for his crime. The angry mob will then prepare to destroy Acadia, out of fear that they'll try to replace more of them with synths. However, if the Sole Survivor has enough affinity with the town's inhabitants, they can be convinced to spare the synth colony.
If the player character decides to eradicate either one of the other two factions, DiMA will not turn hostile but will express disgust at the act of mass murder.
If the player character chooses to either destroy Acadia with the Brotherhood of Steel or reclaim all the synth to the Institute, DiMA will make the last stand with his people.
The following is based on unverified behind the scenes information and has not been confirmed by canon sources.
The year DiMA spent in the cave may be a reference to Plato's Allegory of the Cave, given his philosophical nature.
DiMA is likely a take on the term 'DMA' or 'Direct Memory Access' which is a computer feature that lets hardware subsystems access system memory independently of the CPU. Additionally, Dima is also a shortened version of the Russian name Dmitri.
End of information based on unverified behind the scenes information.