I'd rather not have him in game at all. His life cycle in-game was supposed to be over in Origins, back when he was just a minor NPC.
Why "supposed to be"? Are you a BioWare writer?
"Ascended minor NPC" stories are actually kind of cool to
me, at least, because they help demonstrate that not all the random people you meet in a work of fiction are just interchangeable mooks. This can work really well.
For example, in Matt Stover's book
Heroes Die, the protagonist murders an ambassador for betraying him into the hands of his enemies and for violating a sacred law in the process. One of the ambassador's guards, a character who doesn't even get named, vents his frustration and impotent rage at the protagonist like a good mook is supposed to, but he's prevented from taking revenge and fades into the background. But in the sequel,
Blade of Tyshalle, that guard not only gets a name, but becomes a PoV character and
causes the entire crisis that forms the backbone of the book's plot, out of his desire to get revenge on the protagonist. That book series is one of the more critically acclaimed SF/F series around, and the degree to which Stover's plots interlock like that is a huge reason why.
Bringing Cullen back was similarly cool, in my opinion. Take this random soldier from the first game and use his experiences in that game to help shape his personality, then put him in a plausible place for him to matter for the plot of the second game. What's wrong with that?
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I have already discussed the choice of Cullen as military leader elsewhere, so I don't think I need to rehash all of my points in detail here. However, I will summarize them as such: he is at the very least competent, he commands the allegiance of a significant number of loyal and capable troops, he is demonstrably loyal in all save the most extreme circumstances, and it is highly implausible that any other candidates with similar qualifications would be better - or available.
Post 1: Military CredentialsPost 2: Loyalty and ResourcesPost 3: In-universe Justification for CompetencePost 4: Elaboration on Previous PointsPost 5: Coherent Restatement of Points on Competence