The dative signals a companion as a complement with adjectives and with ὁ αὐτός 'the same'.
The adjectives include ἀκόλουθος [also + gen.] ‘in accordance (with), in conformity (with)’, διάδοχος ‘succeeding’, ἴδιος or οἰκεῖος [also + gen.] ‘peculiar, own’, ἴσος ‘equal’, κοινός [also + gen.] ‘common’, ὅμοιος ‘similar, resembling’, παραπλήσιος ‘nearly resembling’, πλήσιος ‘nearby’, συνήθης ‘familiar (to)’, φίλος ‘friends (with)’.
The personal pronoun ὀ αὐτός ‘the same’ takes a dative of the companion, particularly in cases which concern the destruction of a person or thing.
With words like ἴδιος, οἰκεῖος or κοινός the genitive is sometimes used instead of the dative.
Αἰσθήσεως μὲν οὖν καὶ νοῦ ἀφαιρεθεὶς ἄνθρωπος φυτῶι γίγνεται παραπλήσιος.
A human being, robbed of sensory perception and intelligence, becomes very much like a plant.
ὣς ἐπ’ Ἀχαιοῖσιν σεῦε Τρῶας μεγαθύμους | Ἕκτωρ Πριαμίδης βροτολοιγῷ ἶσος Ἄρηϊ
Thus Priamus' son Hector, equal unto Ares the bane of men, goaded the brave Trojans on against the Greeks.