"si" is a reflexive pronoun in the dative case (simiarly, "se" is a reflexive pronoun in the accusative). Very broadly, "se" means "oneself" or "each other", while "si" means "to oneself" or "to each other".
Some verbs come with these reflexive pronouns attached and it's best to learn them together just like it's important to learn which case a verb requires (although it's usually accusative).
Thus, this verb is: "pamatovat si + accusative" (pamatovat si koho/co). E.g.: Pamatuju si tě dobře.
There is another (slightly old-fashioned) form of the verb, which you may come across: "pamatovat se na + accusative". E.g.: Pamatuješ se na mě aspoň trochu?