I'd like to make a case for Canas usurping Erk, due to his immense utility with Luna. Erk shows up 2 or 3 chapters before him, and has Lyn mode advantage, but while Pent eventually comes along to fill any gaps you may need in the Anima department, Canas is your sole Dark user in the entire game up until the Final Chapter, and has more durability to survive getting cornered if plans go awry. Towards the end, Erk's biggest advantage comes in the form of speed, and that can be remedied by giving Canas a Speedwing, whereas Erk will never be able to use Luna.
And yeah, I am one of the approximately 3 people who uses Canas every playthrough XD, so I might be fanboying a bit.