Coworker calls me “Best Friend”

r/

25M, 20F, coworkers for ~3 years
I’m also in a relationship with my girlfriend (24F), who works with us.

I work in a restaurant and have built a close friendship with a younger coworker over the past few years. We get along really well — lots of laughing, playful banter, and general comfort around each other. About a year ago, she started calling me “Best Friend” regularly. I’ve never called her that back (it feels a bit odd to me), but she’s never seemed to notice. It’s always on her terms.

A few behaviors have me wondering if there’s more going on than just friendship:
• She’ll sometimes mention a “cute guy” at work, but if I ask whether she’d talk to him or get his number, she immediately shuts it down and never brings it up again.
• Occasionally she’ll joke that someone else has taken my “best friend” spot, but if I play along even a little, she immediately backtracks and says I still have the title.
• She doesn’t ever acknowledge my relationship — doesn’t ask about my girlfriend, doesn’t reference her, almost like it doesn’t exist.
• When she and my girlfriend do interact (which is rare), it’s always centered on teasing me. For example, if my girlfriend jokes about me, this coworker jumps in and doubles down. It’s always in good humor, but that’s literally the extent of their interactions.

I’m not looking to pursue anything with this coworker — I’m happy in my relationship — but I can’t help wondering if this dynamic is purely platonic or if there’s something else under the surface. The whole “best friend” thing feels like a strange label in this context, and some of the behavior around it feels emotionally charged.

Has anyone experienced something similar, or have insight into what might be going on?
Would love some outside perspective on whether I’m just overthinking this, or if there’s something deeper happening here.

TLDR: I (25M) have a close friendship with a younger female coworker (20F) who frequently calls me her “best friend,” but never acknowledges my actual girlfriend (24F), who also works with us. She flirts in confusing ways, gets weirdly possessive about the “best friend” title, and seems to ignore my relationship entirely. I’m not trying to pursue anything — just wondering if this is typical friendship behavior or something more. Looking for outside perspectives.