Someone finding themselves either as a Chiss (Star Wars) or as a mutant Quarian in Mass Effect, possessing the abilities of Revan (Star Wars).
Since the story takes place in the Mass Effect universe, the Force would be adapted as an exceptionally powerful biotic ability.
If the main character is a Chiss:
He would be an experiment, the only successful one, the biologically modified son of the Illusive Man, created using Asari DNA, Drell DNA, and of course his father's human DNA.
He would be raised with a single purpose: to benefit humanity. As such, the character would essentially become an assassin or operative, and he would wear an outfit designed to hide his physical traits (Revan's outfit), concealing his blue skin and red eyes. Naturally, the weapons used by the main character would be two lightsabers.
He would initially be a member of Cerberus, likely envied by Miranda Lawson. However, this also means that he and Shepard would almost certainly not share the same views on many matters, so one shouldn't expect them to be particularly friendly.
In short, all of this will lead him to fight against the Reapers.
The MC would know very little about the Mass Effect universe. There would obviously be romance, this is Mass Effect, after all, but it might not be fixed; there could be breakups, just like in the games and in real life, but not sure, maybe people prefer a fixed one.
If the main character is a mutant Quarian:
He would be a Quarian born different from the others, a biotic (with abilities and talent comparable to Revan). The weapons used by the main character would be two lightsabers.
Since the exile from their homeworld, the Quarians have lived sealed inside their suits, prisoners of immune systems too fragile to survive in the galaxy. For them, every microbe is an enemy and every planet a threat. But he is not like the others.
Some members of the Admiralty Board see him as a danger. Others believe he could be a key to the future.
Over time, his objective becomes clear: to find a way to strengthen Quarian biology, so that one day his people may walk freely. To achieve this, he will have to leave the relative safety of the Migrant Fleet (this motivation could perhaps be expanded with additional elements).
In short, all of this will lead him to fight against the Reapers.
Here as well, he would know very little about the Mass Effect universe. As for romance, however, it would be Tali, I think.