BirdTale: Tempest - Teine(Grillby)
Teine/Grillby actually has some established Lore already! They were actually in a relationship with Gaster and the two of them had been planning to bond and have children together... ... then the War happened. In the course of the war Gaster created an alchemical poison that he knew neither the King nor Queen would approve of... but deployed it anyways hoping it might end the war then and there. (It didn't) This basically constituted a horrifying war crime and Teine immediately broke off their relationship, unable to look Gaster in the face without thinking about what he had done. Their prior relationship became and uncomfortable subject that was 'not to be spoken of', and eventually forgotten by all but the oldest boss monster that still remained from the war. It is one of the reason he allows Sans and Papyrus to have a near unlimited open tab at his tavern. It's not just that they are close friends... but in Teine's eyes they are the children that might have been if Gaster and himself had not separated. Neither Sans nor Papyrus are aware of this prior relationship of Teine's with their 'father', though Sans suspects they must have had a close friendship at least! Note: While Teine did fight during the war he preferred (and felt more useful) functioning as a medic.