Is “together” いっしょ (issho) or いっしょに (isshoni)?

I always thought “together” was “isshoni”, but I’m playing J-crosswords and the answer to the clue “together; at the same time” is “issho”.

So it seems like “ni” in “isshoni” is just a particule?

Is “issho” ever used without being followed by “ni”?

*** RANT BELOW ***

_For some reason, I’m very embarrassed to be asking this question. Probably because “isshoni” is one of the first words I have learned and it stuck in my memory even through the 10 years I went without practicing Japanese, and it never crossed my mind that “ni” was just a particule…_

_I just feel betrayed by this word now._

_Although, let’s be honest, it’s probably just displaced feelings for my guilt about to putting more effort in studying Japanese_ 🙁