I'm 90% sure quarians aren't coming to Andromeda, I'm 99.9% sure we aren't getting a companion even if they do. It has nothing to do with any logical in universe reason (which is why the krogan are in even though they're the most counterproductive thing possibly imaginable for a venture like this), Popularity rules, and people at large (i.e. outside of fan communities like this forum) hated the quarians, particuarly in ME3. Tali was reasonably popular....because her primary character arc was basically all about recognizing the "stupidity" of and abandoning her culture, becoming a quisling, and in general acting like an individualistic human. The Admiralty (who are better representations of the species at large) by contrast, are some of the most widely hated characters in the series. Even if they are included despite this, it's far too late in development to start proposing ideas anyway, but I'll play along anyway because I've nothing better to do but shitpost on a forum about a game I probably won't get.
I suppose my ideal would be a loyalist, but one not so friendly and benign as Tali was. Individuals like Daro'Xen and Han'Gerrel excluded, I always thought the quarians at large were a bit too friendly to outsiders given their high level of species-nationalism, fairly undemocratic system of governance which had no problem using show trials and propaganda to shape public consensus, and the manner in which they were usually regarded and racistly propagandized as subhuman criminals in Council space.This person should initially resent and detest the Council societies as much as those societies hate the quarians. Perhaps they could be some sort of intelligence agent from Xen's special projects arm of the Flotilla, implanted in the Ark program to conduct espionage and steal tech secrets that could be useful to the Flotilla (oh, so some of them really are thieves?). The reveal of their status as a spy could be either before the game (have the character captured and make some sort of deal to help the PC/ project) or midway through it (have them pose as some random pilgrim or exile hired for tech expertise). Morality would be neutral and utilitarian, not particuarly constrained by any particular values apart from that of improving the station of their species, but not deliberately callous or comically brutal as biower tends to portray such characters (this is probably too much to ask). They would be as emotionally volatile as quarians are typically portrayed and characterized, but perhaps not wear it on the surface as much, a bit like Xen, or even like Golo from the novels (in temprament, not in being a traitorous, psychopathic murderer).
They could be male or female, and I don't really give a shіt about romance compatibility, but if it is included, I don't want it to end up shaping the characterization like it did with Tali (she wasn't originally intended as a cutesy little kawaii space girlfriend, but that's what she partially became due to fanservice). Combat class would be infiltrator, but preferably one who carries short range weapons and specializes in CQB (quarian's hat in terms of infantry combat is supposed to be shipborne fighting in confined spaces), possibly with a knife or other semi-practical melee weapon as well just as a stylistic callback to Tali's (which got too little use IMO). Closer to Kasumi than Garrus, in other words, so we don't get just another sniperfiltrator.
I'd be open to pretty much anything, though, because anything would be better than nothing. Just don't break the lore and have them all suddenly stop wearing envirosuits (which in addition to being nonsensical according to established lore would pretty much defeat the entire purpose of their initial design concept and render them into little more than Star Trek aliens) or making friends with toasters (if they're also there) even if the divergence point is pre-Rannoch, just because reasons. The suit in particular is a symbol of quarian fortitude, a visual representation of their will to survive, adapt and overcome. I'll start flippin' desks if biower includes quarians, then goes on to ruin them all anyway.