Learn Finnish: Me halasimme kavereita.

You would replace kavereita with toisiamme. Because the verb halata always needs a partitive object, we take the plural partitive form of toinen, which is toisia-, and add the 'we' possessive suffix -mme.

Why is the object kavereita in the partitive case?

In Finnish, certain verbs express emotions or actions that don't have a clear endpoint, and they always require their object to be in the partitive case. Halata (to hug) is one of these verbs, so we must use the partitive plural form kavereita.

How do we get the past tense halasimme from the basic verb halata?

Halata is a type 4 verb. To form the past tense for this specific verb type, the present tense stem halaa- changes its end to become halasi-. Then, we simply attach the 'we' personal ending -mme.

Is the word me strictly necessary at the start of this sentence?

No, it is completely optional. Because the verb ending -mme already clearly means 'we', you can just say Halasimme kavereita. Dropping the 'I' and 'we' pronouns is very common and natural in Finnish.