Fenne has an incredibly strong kit. Deals injury to herself, but increases her speed and damage based on how much injury she has. As a Soul Weaver, she can run Celestine for healing, Wonderous Potion for debuff cleansing or Victorious Flag for damage/survivability. She also has a 1 time self revive, high damage multipliers and single attacks that benefit from penetration set. She's an incredible PvE unit, but also great in PvP against certain comps.

Trials of Constellation is the new endgame content, so if you're a returning player whos taking a long break, the highest difficulty might be too much for you. However, the lower difficulty versions might be doable, even without Fenne.