In a dictionary, verbs that can take a direct object will be marked "transitive verb" or "v.t." and those that can't will be marked "intransitive verb" or "v.i."; many can be used either way and will have two listings.
Here, "ressembler" is an intransitive verb only, so any object must be indirect (using "à", unless the object is a pronoun, in which case it is placed before the verb and the "à" is dropped).
This is really a dialect expression, East-Flemish actually, somewhere around the city of Gent, when people talk of someone in particulary, about his escapades, they can end with : "Hij is een ei !" , it's exactly like in the movie Goodfellas when the Italian-American gangster said: He is a funny guy !