If Marcus has been prevented from capturing Isobel and destroying Last Light Inn, and if the Nightsong has been freed from her cage to wage war against Ketheric Thorm at Moonrise, Olys will take the place of Harper Naysa as a failsafe in the event that the latter has been killed before the Assault on Moonrise Towers. Talking to Olys has him nervously reveal that he wished he'd used the latrine before going to battle[1].
After the main battle, if he manages to survive the fight, he will guard the first floor of Moonrise Towers with the remaining Harpers. Talking with Olys will have him show signs of nausea from seeing a tadpole crawl out of a corpse[2], and he will plead the party to finish the fight[3].
If killed during the Assault on Moonrise Towers, a replacement Harper failsafe (either Grenywald or Lanni), or a generic Harper Archer will take his place as a Silver Harp Squadron summon.
Olys has four level 1 spell slots, and two level 2 spell slots, despite having no spell to cast.
Unlike Naysa, who is unable to attack twice during a turn despite having the Extra Attack feature, Olys is not affected by this bug.
References
↑"Wish I'd used the latrine. Always need to, before a battle." - Olys
↑"Gods above, the tadpoles. I think I saw one crawl out of a corpse."