intersum

Intersum, interesse, interfui is inter plus sum, and it inherits every irregularity of sum unchanged. It leads two lives. Personally it means "be present at, take part in" and takes a plain dative: concilio interesse, to attend a council. It also keeps the literal sense of lying between two things, from which comes "inter A et B interest", there is a difference between A and B.

Impersonally it is the awkward one. "Interest" means it matters, and the person who cares goes not in the genitive you might expect but in the feminine ablative singular of the possessive: mea, tua, nostra, vestra interest. How much it matters goes in the genitive of value, magni or parvi, or in a neuter accusative like multum and nihil. English interest comes from this impersonal sense, by way of the third person form itself.