Well Odette appears in both categories. As well as buffer due to her non-AW skill. How I'm personally handling it is that if I choose a unit for a role I won't generally have the for another role. For example, take Clissa. I have her as one of my three ganking units. She also appears in Anti-Rush and Ramp. In Anti-Rush instead of having Clissa for that I choose a Rogue, Monk, and Ninja instead (.... stares at all the fox spirits... I'll have you one day!), and for Ramp I choose another Valkyrie to take the role. Allowing that Valykrie to become a Geirolul better fitting that role, and Clissa to become a Reginleif to better fulfill the ganking role.
I think there was only one case in which I had a single unit fill multiple positions and that was simply because I didn't see any value in the other options. I believe that was Koyomi as both an Armor Breaker and AoE Rear unit. Which I suppose the example Odette would do that same thing.
It was originally named Defense Breaker, but that didn't make sense. Since the defensive stat in Aigis in named Defense and not Armor. So instead of it breaking Magical and Physical Defense, it is breaking Magical and Physical Armor. If that makes any sense.