BlazBlue:
Alex would probably end up randomly wandering around then heard about some Grim Reaper and see what the hell is going on since he felt confused on why the world is so screwed up, him not knowing that the Dark War happened and 80% of the world is gone. Though he would get yeeted by Hakumen if he comes across the guy first since the Power of Order would sense Alex Mercer and Hakumen exists to destroy evil. Not letting the guy get the talk in Hakumen pretty much stomps Alex and Alex would get annoyed and assumed he can consume Hakumen but failed because Hakumen is not organic and can cut reality thus ending Alex's existence. If this Alex was after Prototype 1 then he would probably end up consuming various people and helping Ragna along with the way and might end up being a huge pain in the ass for Terumi, Relius and Izanami but in the end Ragna and Alex teaming up would be an awesome endgame battle against Susano'o. Though if this was Alex in Prototype 2 then Alex starts wrecking shit up with his virus until Kokonoe and Relius teams up though reluncitly and devised a cure, then has Hakumen and Ragna corner Alex and murderstomp the poor guy, to death.
In fact Alex would get deleted anyway by The Master Unit if she finds out about Alex and in fact Alex has NO WAY of getting anywhere near the realm that The Master Unit is in and in fact Alex has no counters to universe erasure, time changes, or going into a reality warping dimension and many more stuff. Plus Terumi is a ghost so Alex can't touch ghosts, and most of the characters are immune to infection thanks to them being able to handle Arakune, such as Noel getting eaten and surviving Seither surrounding her. In fact if Alex tries to consume Arakune, the Seither itself would warp and mutate Alex Mercer into another Arakune and Alex lost his mind and then is a mindless monster to get yeeted by other high tier characters.
So these characters would find and yeet Alex with ease: Ragna the Bloodedge, Noel/Mu-12, Jin Kisaragi, Hakumen, Amane, Rachel Alucard, Naoto Kurogane, Es, Makoto, Terumi, Relius, Master Unit, Susano'o (If Terumi sneaks up on Hakumen and regains the Susano'o Unit and becomes Susano'o once more), and Tsubaki.
Shin Megami Tensei:
Alex would probably end up randomly appearing in some hellhole (depending on what version of Tokyo in the Shin Megami Tensei games) and having to deal with demons, angels and other crazy shit. Alex would probably end up consuming various people trying to survive at first then began trying to help any of the Shin Megami Tensei protags though he would probably choose the neutral path and say screw the law and chaos he is doing he is own thing and the protag (depending) would probably chose as well. Though if it was Shin Megami IV: Apocalypse then he and Nanaki would destroy God and everything due to their nihilism. If this was the evil Alex in P2 then he starts wrecking shit up with his virus until he comes across demons, angels and Gods that would get pissed off by Alex's personality and Alex ends up deader than dead. So either way Alex is screwed beyond repair if he goes any path.
Devil May Cry:
Alex would live his life normally and not want to get involved until demons pop up and would have to deal with it though might get into a fight with Dante or Nero and end up getting killed by mistake by the demon hunters. If it was in DMC 1, and 3 then Dante would have a massive help since Alex would be consuming demons while Dante gets Demon Arms and kicking ass. If DMC 4 then I feel like Alex would help Nero but also mock him though if he fights Nero then Alex would get stomped pretty badly. Also if this was DMC 2 then Alex would probably be some help though Dante would speed though the place like in canon and if Alex tries to kill Dante then again Alex gets yeeted into oblivion. Finally if DMC 5 I would feel like Alex would be sick of the shit and help though would not probably allow V to rebond with Urziel thus ending Vergil's return, leaving Nero never knowing who his father is. If evil Alex appears and starts wrecking shit up where this causes Dante to go full power mode to stop Alex realising how much of a threat he can be and stomps Alex.