There was that one game, ugh, can't for the life of me remember the name... Red cap does sound familiar, but like everyone's said - not Mario. Was it something like 'Monster Bash'?

lolol it's obviously Mario, you just forgot what a Nintendo looks like. PC gaming in the 90s, as if!

Everyone's all about Commander Keen, Jazz Jackrabbit, or Duke Nukem, but you my friend are CLEARLY describing The Adventures of Captain Comic. It's got potions, keys, and the main dude runs around in a red suit that can easily be mistaken for a red cap. Used to play it on my old 386 like it was nobody's business.

Sounds like Secret Agent to me. You got this dude in a trench coat and fedora (kinda looks like a cap maybe?), and you definitely had to gather keys to unlock doors. Potions? I seem to recall there might've been something like that too.

lol potion n keys?? red cap and not mario... dude idk, but now I wanna play whatever game this is 🤣

Your description could match a few games, but the red cap detail is intriguing. One game that fits your description quite well is The Adventures of Captain Comic. In this game, you control Captain Comic and wander across different planets, collecting items, including potions and treasures. Originally for MS-DOS, this game was one of the earliest side-scrollers for the PC. And yeah, the titular character wears a red suit, which might be what you remember as a red cap.

hmmm red cap but NOT mario, on PC... sounds like Jazz Jackrabbit to me, except the cap is green in that, oh well lol. That game was sick though, fast-paced and those tunes were catchy as heck

This is ringing some bells... Are you talking about Commander Keen? That dude with the pogo stick and helmet? It was pretty rad, and had all the pixel goodness. Def had to grab keys and stuff. Does that jog your memory?