Hit Points: C
Magic Points: S
Strength: C
Defense: D
Magic: S+
Magic Defense: S
Intelligence: A+
Wisdom: D-
Will: D
Dexterity: C
Speed: C
Stamina: C
Health: C-
Other: Super high magic stats across the board! Howl’s teacher notes he’s the best student she ever had, and he eclipses the king’s wizard, who is supposed to be the best of the best, by kind of a lot. Physical stats, not so much. Lots of average, here, the notable exception being below-average defense; he sort of crumples to the ground in a faint when pain happens. He is also bed-ridden and complains of dying from a cold, but as that seems to be mostly his propensity for drama rather than actual health deficiency, he only gets a C- for it, since he seems to get sick fairly regularly but with nothing too serious.
High intelligence; he’s clever and creative and learns new things well. He can shortcut his own spells and seems very well-read in general. According to the book's chronology, he has completed his doctoral thesis and moved to Sophie's world by the time he is twenty-two- which means he either entered university very young, took super-accelerated courses, or a combination of the two. Book smarts + an active mind, here, and though he doesn’t remember what he knows in canon yet, the base cleverness is there, and the aptitude for learning.
As for wisdom, it is godawful. In canon, he does things like spend all his money on clothes, inadvertently leaving nothing left for food, or take hours in the bathroom getting ready to go out only to use a spell to disguise himself over the careful preparation anyway. His priorities are a bit mind-boggling, and his common sense is ridiculously low. Low will, as well. He’s kind of a coward, and he tends to slither out of anything unpleasant rather than actually face it or deal with it.