I finally designed them... Yes, they're aesthetically based on existing characters from other two series -- it's just their appearances that i'm using as reference (i did not watch either of those shows sorry)
Meet Yuna [ 佑成 ] and Makoto [ 洵 ] -- Rui's cousin and cousin-in-law respectively~
Yuna is 5 years older than Rui, so when her parents took care of him, she was all around him, trying to be supportive and help him. But Rui was suffering a huge trauma and isolated himself from the others, keeping only the most basic contact with her and her parents. They couldn't do anything without Rui's wishes, so they just offered her shelter, education and food.
At this point, Yuna was dating a friend and classmate named Kouda Makoto -- a few months older than her -- who had helped her deal with the frustration she felt, thanks to the belief that she was useless if she couldn't help her only cousin deal with all of "whatever happened to him".
Makoto and Yuna had this silly, sweet dream of opening a cafe, which came true in January 2012. Rui was ready to move out, so they decided to hire him to work at the cafe and help him get money to buy a small apartment for himself. Some of the things for Rui's apartment were Yuna's and Makoto's help (plus Yuna's parents, too).
Despite everything, Yuna's personality is very motherly and has a lot of responsibility in it. She's always worried about how naive or reckless Makoto and Rui are. She may sound cold or harsh, but she's not. She's more blunt and brutally honest, but never mean or sarcastic. She's quite friendly and outgoing, and always tries to keep the Coffee Shop in a good mood.
As for Makoto, he's actually the first person he's felt like a "father figure" to in a long time. Makoto is bubbly and gentle, but at the same time a walking mess. But he's calm and patient and tries to help Rui overcome every obstacle. Ah he's a bookworm and an airhead sometimes.
Ah and they're... a Power couple. Yeah.