Learn Turkish: Senin adaşın dün beni aradı. - Your namesake called me yesterday.

It is formed by combining ad (name) with the fellowship suffix -daş. In Turkish, when this specific suffix is added to a word ending in -d, one of the matching consonants is dropped to make it smoother to pronounce. So, ad + -daş becomes adaş.

Does the -DAş suffix follow vowel harmony in this word?

Yes. The capital letters in -DAş indicate that it follows A-type vowel harmony and consonant mutation. Because ad has a back vowel, the suffix becomes -daş (rather than -deş). If the root ended in an unvoiced consonant, it would mutate to -taş or -teş (like in meslektaş).

Since adaşın already means 'your namesake', why is Senin included?

You could simply say Adaşın dün beni aradı and it would be perfectly correct, as the -ın suffix already marks 'your'. Adding the full genitive pronoun Senin before it just adds emphasis, stressing that it was your namesake who called, as opposed to someone else's.

Why do we use the accusative beni here instead of the dative bana?

The verb aramak literally means 'to search for' or 'to seek', and by extension 'to call on the phone'. Because you are 'seeking' someone (even over the phone), it takes a direct object. Therefore, it requires the accusative case beni (me) rather than the dative bana (to me).