Jas is not a benevolent character. When insulted by the player, she just tells him that life is an aberration and will end before wiping the multiverse (this was a non-canon ending to the confrontation). She will probably lead with her outright attack, which shouldn’t do much to Mesprit considering her durability.
Mesprit will probably also try to attack, although she will soon realize this will be a bit harder than expected because of Jas’ abstract nature.
Jas doesn’t have many emotions to drain from the get-go. Confusion would be effective if it wasn’t for the nature of Jas. Jas will probably try to just kill Mesprit, and when that doesn’t work she will probably try to drain her anima.
Anima absoption drains away the life, mind, and soul.