When I was in school I learned 여보 was a contraction from 여기 보세요. Essentially spouses tell each other to pay attention. The only "rule" I was ever told about it was that it is weird to hear newlyweds use the term; instead, couples married ten years or more saying 여보 was the norm. Incidentally, it shares its root with the telephone greeting 여보세요. Or this was all old wives' tales my teacher made up to keep class interesting.
It's not a pet name, like 자기야 or something, so it would be a neutral "filler" name like "honey" or "hon". At least my own feeling about "honey" (and 여보) is neutral, vs darling or dear. I guess it's subjective. It probably depends on how you say it. For instance, I feel like this about "honey" and "여보" because I usually hear them spoken in neutral tones.
The Korean pet names I know are "자기야" which sounds like "baby", 오빠 which is literally "big brother" so it's kinda gross, and 애인 which sounds like "sweetheart".
오빠 / 누나, while literally meaning "big brother" and "big sister" when translated to English, are actually very differently used as opposed to other languages.
E.g., A girl could call a guy (older than her) who she's close to 오빠, and it's not weird/gross in Korea (whether they're close friends, dating, or actual blood relatives). You shouldn't really think of it as only meaning "Older brother" because honorifics like that (Including 언니, 누나, 형, etc.) have lots of uses depending on your relationship with someone.
(This was kind of haphazardly typed out so I hope it made sense/was helpful)