This whole comment right here.

I was thinking the romantic interest thing right out of the gate. I like your emotional need take, too. Either way it's not work appropriate esp. if OP has communicated their disinterest before.

I also like your direct communication prompt.

While you're right about how HR works/how to go about it, I find HR a hammer when a finesse tool might work. OP could ask their manager how to deal with it. Not only does this put it on the radar, but it's a bit less official, and might yield the gray zone outcome OP is looking for

To add to this, OP may want to put their thoughts down in writing. An email or memo or note or anything as long as it's not a text or other personal non work form of communication.

Lay it all out, over write it, put too much detail and then pare it down to what OP believes to be acceptable. Have some copies of it.

Give them the written missive that boils down to "leave me alone" Make a note of the time and date.

If the co-worker keeps being a pest, go to HR, with a copy of the missive, and a note of the date it was given to co-worker so that co-worker cannot wheedle their way into making it about them trying to "help" OP.

Also, fuck every co-worker who pulls unprofessional shit like this. It's abhorrent and borderline harassment.

You going to HR can also take place of talking to him yourself. You talk to HR and ask them to tell him to let you work, they talk to him so you don't have to try and navigate his bullshit and you get the issue noted to support your side of things.

This is how humanity dies.