In the Shin Megami Tensei, usually there is a device called COMP. This device lets the user control, summon, and fuse demons. It is usually found in the main line series, so series like Persona and Raidou lack this device. Although I think Raidou did not have this device because his era’s technology has not reached this point.
Almost every game has a different COMP. What I mean by that is the looks, the origins, and functionality. The capability of the COMPs differ from game to game as well. For example, in Devil Survivors, you and participate in Demon Auctions, but this cannot be done on COMP from SMT IV.
For example, in Devil Survivors, the COMP looks like a Nintendo DS. It was built by Naoya, one of the characters that joins the party. The COMP allowed the user to control the demons they could summon, but some people were not able to use it properly. And many people were able to use it as long as they access.
In SMT Devil Summoner, a Gun-COMP or GUMP was used. It is a gun fused with a COMP. Since I did not have a chance to play the game, I cannot say too much regarding this device.
In SMT IV, the COMP was treated as a sacred device reserved only for the samurais. It’s origins was not very important, and it allowed the user to cast spells as well as fuse demons.
Probably the most powerful and fun part of the COMP is the Demon Summoning Program and the Demon Fusion abilities. In SMTIV, the Cathedral of Shadows was an app, that actually hurts to type, so fusion can be done anywhere. Although it is very convenient, the actual building was really cool.
Although it not a COMP, in Strange Journey, the Demonica suit served a similar purpose. It helps the user to communicate with the demons as well as protect them from various attacks and environments. A very similar suit also made an appearance in SMT IV.
I must have missed some stuff, let me know if which ones I missed!