Hi!
DUESSEL
Duessel.
Duessel is interesting. He's abso-fuggin-lutely redundant on Eirika's route, but he can save your ass on Ephraim's route. This isn't to say he's useless on Eirika's route, per se, but by the time you get to C15 you probably (definitely) have a favorite Cavalier who's probably been promoted - f'example, 20/1 Kyle is already better than Duessel in most measurable ways. Thus, in keeping with the rest of the list, I shall focus on his better route - Ephraim's, where he joins a full five chapters earlier. Ephraim's route, Chapter 10 is not a difficult chapter. You're forced to recruit Duessel anyway, so you're going to - no points off for being a pain in the assRennac. C10 he comes in as a --/8 Great Knight, which is probably higher than everybody in your party bar a heftily abused Seth or a unit that you arena ground to 20 in Joshua's chapter (done it without savestates. never doing it again.) So it's safe to say that Duessel is your best unit when you pick him up.
However, the biggest factor giving Duessel the points is Ephraim 11. Ephraim 11 is better known as the Pirate Ship, and it's fucking stupid. Reinforcement fliers from everywhere wielding lances combined with reinforcement fliers everywhere using magic and also fog of war plus L'Arachel who may very well die if you turtle too hard, failing you the mission (citation needed) - did I mention Fog of War? Oh, and there's a BOSS Deathgoyle who spawns back there too. The only other units besides Duessel who can reliably hold back these tides are Seth and maybe a babied Vanessa. Everybody else will either die to the magic, or die to the lances. Duessel? He stands and laughs. Fuck, he could probably rescue your healer and still fight them back, with 1-2-range WTA in the form of Hax Axes.
Sadly, that's Duessel's peak. He provides an immeasurable amount of support on the Ship, and then he becomes a second Seth - not necessarily a bad thing, but. Afterwards, he becomes a rather overleveled but slightly below-par Great Knight. GK is a good class, fortunately, so he gains back for that, but once any of your other Cavaliers start hitting his levels he loses all his potential due to mediocre growths and poor bases for his inflated level. What's this mean for his rating? Well, he helps massively for Ephraim 11-14, and then he's probably phased out due to mediocrity - but not badness. As such, I think it's fair to give him a
8.0
but those pretend points you've been subtracting from Innes and Saleh and Cormag for their bad routes, subtract double from him.
EDIT: Fuery's playthrough log gave me useful knowledge - L'Arachel does NOT fail you Ephraim 11 if she dies.
EDIT2: Duracell is now officiall an 8.0.