Hi there, Interesting question. Over the years many fans specualted that Chris and Jill were a couple, I myself was convinced they were. They are extremley close and very protective and loyal to one another!
However all Chris/Jill fans living in hope of having a romantic relationship between the pair confirmed were doomed to be dissapointed.
In Resident Evil 5 Sheva ask's Chris about Jill, after picking up on his deep affection for her she ask's "You two were close?" and Chris simply responds "We were partners" so Unfortunatley they were never together romantically.
- "We were partners" seems quite an ambiguous response to me, but I suppose different people will interpret it differently. But Capcom has neither confirmed nor denied anything in regards to Chris and Jill before 1998, so there isn't really a concrete yes or no on the matter. Ghost Leader
If you play RE2 and look at Jill's desk in the STARS office your character will comment on a picture of Jill's boyfriend, so if they were in a romantic relationship it would have had to happen after RE 3. As for me, I like to think of them as just really good friends.
There is also the matter of Jill's Normal ending for the first game, which I believe is actually also the same as Chris' - I'm not sure - where the two are along in the helicopter, and they hold hands. The picture is explained in the novelizations as being of her father, but those aren't exactly canon. All in all, it's really up to speculation on the part of the player. I personally think they were involved to some degree, at some point.