SPOILER ALERT.....
It's been a while since I read the novel and never finished but I got quite far. Not too sure if I remember everything correctly so take everything with a grain of salt
The newest esper that came for guiding is MC's ex-esper partner (not intimately/romantically involved).
The Central wanted to exploit MC's guiding ability to create alliance with foreign nations(?) and the ex-partner overheard their plans. He loves MC and doesn't want that to happen. He planned his own death and managed to take over current esper body (don't remember how, but there were a lot of experiments done). He also planned MC's death to awaken in new body so that he can take MC and leave the Central. I wouldn't say he is "evil", just went about protecting MC the wrong way and causing a whole lot of pain :(.
Original MC and ML were never a couple/lovers in the past. They were paired (MC was kinda forced to become ML's partner because there were no guide strong enough to calm his awakening+ he feels bad for the kid) after the "passing of his ex-partner". Original MC did have to use physical guidance with ML, but never went beyond kissing. MC (early 30s?) viewed him too much as a kid (19-20?) so no romantic feelings on MC side. ML however, has one-sided feelings for MC.

I need spoilers anyone pls