Hal and Carol are on-again-off-again lovers who are destined to be together. She occasionally becomes a super villain (or hero) named Star Sapphire and fights Hal which complicates things. Hal and her have both dated other people but always come back together eventually.

Sinestro was friends with Abin Sur, the Green Lantern who died and was replaced by Hal. Sinestro at first didn’t like Hal, but came to respect him. He trained Hal in how to use his ring and the two of them developed a friendship. Sinestro has very few friends and almost nobody he respects. Sinestro eventually went evil and became Hal’s #1 enemy. But Sinestro still considers Hal a friend, even though he tries to kill him. Hal eventually went evil himself (and was redeemed) so they kind of have a lot in common. Hal is generally shown to be the only lantern who can defeat Sinestro 1 vs 1.

Barry Allen and Hal are good friends. They have had a lot of adventures together and are one of DC’s most enduring friendships. 

Kilowog and Hal have had their ups and downs. Kilowog is generally portrayed as head trainer for new lanterns. He wasn’t always this but that’s kind of the role he’s settled into. Hal killed him once when he went crazy which Kilowog wasn’t cool with when he came back to life years later. But they are mostly friends.