partnero

A borrowed root, whatever it looks like. Partnero is partner- plus -o, in one piece. It is not parto plus anything, and in particular the -er- a learner might think they can see is not there. -Er- is the suffix for one small element of a mass, and it is the reason sablo gives sablero (a grain of sand) and mono gives monero (a coin). A partner is not one small piece of a part. The apparent seam is an accident of the English spelling, which was itself an accident: the word started as parcener and drifted towards part because speakers assumed the same thing learners of Esperanto assume.

Two senses, both current. The commercial one, a co-participant in a firm, a project or a coalition, which is what la partneroj de la entrepreno means; and the personal one, the person you live with, which is now the usual way of saying it without specifying marriage or sex.

The native alternatives. Kunulo is a companion, made from kun plus -ul-, and it is broader and warmer. Kunlaboranto is a collaborator in the neutral sense, kunkomercanto a business partner, and for the domestic sense kunvivanto exists and is used. Partnero beat all of them for the same reason nuklea beat atomkerna: it is what the international press writes. The verb partneri, to be somebody's partner, is a back-formation and is fine.