At Lvl 4 take whatever you prefer, I prefer Inner Light for the burst healing and because I frequently hit Lightning Fury, reducing Inner Light's cooldown. Plate of the Whale is a good choice as well for sustained healing, and Ring of the Leech is very situational and will either be strong than the other choices, or do little to nothing.

At Lvl 4 Ring of the Leech will be a good choice, Inner Light is immense burst healing but wasted if you don't hit or use Lightning Fury often, and Plate of the Whale is strong seeing how you as Auto-Attack Cassia, should constantly stay on the move between auto-attacks, but Ring of the Leech will be the strongest.

At Lvl 10 it's whatever you prefer, if you wanna make a pick for your team, take Valkyrie, if you just wanna have more damage take Ball Lightning.

At Lvl 13 if you took Inner Light, take Thundergod's Vigor, if you wanna be able to execute, and "stay safe", take War Matron.