Osilus

Osilus is a major character and party member in Act One of I Am Still Alive. He is a scholar who saves Emily after she fights The Entity and is sent plummeting down into the Mansion Depths.

He is initially presumed to have died in the collapse of the Veril Spire, but a scene after the credits reveals that he was saved by the Old Hero.

Emily
Osilus finds Emily after she is killed by the Entity, and due to his knowledge of matters related to the Wardens, he realizes that she may find a way out of the Realm Beyond Life, and opts not to bury her. She eventually wakes up and, after some exploration, asks Osilus to tag along with her on the trek to Veril.

Osilus is initially surprised by most of Emily's actions, but the two grow close very quickly, dishing out playful insults at each other often.

Emily expresses sorrow at being forced to leave Osilus to (presumably) die in the Veril Spire's collapse. Once Osilus is saved by the Old Hero, the scholar asks the latter if he's seen Emily. The Hero states that Osilus was likely the only survivor, and that he hadn't seen anyone else, which clearly saddens Osilus.

Lyght
Lyght is distrustful of Osilus and Emily at first, but after the trio bands together, he decides to keep an open mind. Despite this, he seems to struggle with mood swings, getting angry at the other two at varying intervals. Emily and Osilus tolerate this, treating Lyght with kindness regardless. Despite this, Osilus seems to have a clear bias towards Emily, believing her to be capable of saving the world, as opposed to Lyght, who is doubtful. This leads to a miniscule amount of tension between the two.

Defeating Egon, however, drives Lyght over the edge. He becomes enraged at Emily and Osilus, blaming them for getting his hopes up. Emily simply feels bad, partially because of Lyght's words, and partially out of sympathy for him. Osilus, however, defends Emily, stating that despite pitying Lyght and understanding where he's coming from, he believes it isn't an excuse to lash out at Emily and himself.

Despite this, Osilus regrets his actions after Lyght leaves the party, and is willing to search for him while Emily continues onwards (due to him not having a way out of the Spire.) After the collapse, he asks the Old Hero if Lyght survived, and is surprised to learn that the Hero knew who he was talking about. The Hero then states that Osilus was likely the only survivor, and that he hadn't seen anyone else, which clearly saddens Osilus.

Old Hero
Osilus and the Old Hero only interact briefly, but the former is shown to be grateful to the latter for rescuing him.

Trivia

 * Osilus is the creator of Pathosorcery, which he wields in battle. His combat animations depict him writing out the scrolls (which are stated to have finite uses, and must be re-written every time a Pathosorcery spell is to be cast.) These scrolls use the ancient language of the Wardens, a.k.a. Latin.