Reina

unfortunately happens when mathias is drunk. it looks like he's wearing the same outfit in the screenshot from the episode 9 description, which is: "mathias wakes up from a party and remembers nothing. he slowly remembers how everything went wrong."

timewise, the party seems to happen after erik hooks up with sara in episode 8 (maybe even the same night, since the episode 9 description reads that it's about mathias remembering what happened at a party).

so, my theory is that mathias, happy and drunk at the party, just glad to be somehow hanging out with erik, kisses him. felix and the guys (or random guys but this screams felix) see mathias drunkenly kiss erik, and then draws the dick on mathias face after he passes out from drinking too much, as the episode 9 title is: have you looked in the mirror?