-The boy akin to an impassioned bard recited his stories
-The girl similar to him stood still.
These adjectives (italicized) and others similar are always placed next to a prepositional phrase (bolded) of some sort. Do the adjective + prepositional phrase make an adjective phrase? And what is the prepositional phrase, by itself, modifying?
Yes, those adjectives and their preposition phrase complements (they are allowed by a certain subset of adjectives) do form adjective phrases. These adjective phrases are modifiers in noun phrases in your examples where they modify the head nouns boy and girl.
That they are part of adjective phrases is supported by the fact that these adjective prhases can be used predicatively:
He described the boy as akin to an impassioned bard.
She was similar to him.
Note that the adjectives in question are not always placed next to preposition phrases - that is, they do not always require a preposition phrase complement.
The dog and fox are closely akin. (Merriam-Webster)
no two animal habitats are exactly similar (Merriam-Webster)
