click star up and this menu will remind you that you need soul splinter of the hero and two types of runes that could be obtained from the rune summoning chamber. I screenshot this after performing the starnupbfrom 1 star to 2 stars Jarvis, so this screenshot is for his next star up to 3 stars.
these are the two runes needed to up star the dashing Jarvis to 2 stars. Make sure the runes are square and green and blue one plus one. They are three star runes if you are familiar with. So according to the previous screenshot, his next star up will need the hexagonal runes of the same colours.
this is Jarvis soul splinter. I never bought any. I have got them all from events such as King's Trial as free gifts.
Example of a starred up Jarvis. 1 star to 2 stars with upgraded skills. Congrats my dear (mahaha! and you cannot rest until I level down 15 camps in this 2 hours. As you were!)