Mordred or Nero bride

Mordred if you need a saber for farming. Nero if you need a support character for challenging content. It depends on what you need, but I'm partial to Nero.

Nero bride is better for harder content. Level her first, and if you need a farming saber you could always level Mordred later.

Nero.
She’s great support, especially if you’re trying to do a solo run. Single-target NP is generally better than AOE.

Nero Bride since she has a lot of good support skills. She can heal anyone, boost their NP gain and attack. Mordred is more solo strength