In "Game", Kouga picking out the correct Kaoru in a sea of doppelgangers simply by thinking about every expression of her's that he has memorized and then matching the real Kaoru's sad expression with the one she had the last time they spoke that day when he upset her.
Followed shortly by him restoring her soul to her body and then allowing her to rest against him as they sit on the stairs, with her murmuring "sorry" to him and him looking surprised and awkward.
Kouga: In the beginning, she was just Horror bait to me. But then... I realized something. Her existence...is the reason I fight. I will protect that girl with my own hands. Before the hundredth day comes, I will definitely do it. Even if it goes against the ways of the Makai Knights, if I can make that girl's life shine a little.
Then at the end, when Kaoru is there waiting for him and the episode ends with the two of them smiling affectionately at each other. The events of the next episode, though, make this scene somewhat bittersweet.
"Life" is full of them. The entire scene when Kouga encounters Kaoru's soul in the forest of Glen while she's on her way to the Underworld. Beaten and ready to give up, Kaoru tells him to let her go already and that she has no wish to return to the land of the living if it means her life is only going to be filled with more danger. His response to that is an Armor-Piercing Slap with a dash of Get a Hold of Yourself, Man!. He then grabs her in a fierce hug and finally apologizes for failing to protect her before as well as re-kindling her will to live:
Kouga: Kaoru, you still have time. I want to see more of your paintings. So... Please don't lose the will to live [....] I will definitely be back. Believe and wait.
Kaoru and Kouga's first date in "Magic Bullet". Between Kouga's partial case of Cannot Spit It Out, him showing her the village where he grew up, and then putting his arm around her in his first casual show of intimacy and affection, it's all pleasantly calm and sweet...and then it all went downhill.
In the fourth episode of Makai Senki, the dealer tells Kouga, when the latter requests more money to continue his bid against the Horror, the dealer replies with something like "Then we'll take what you hold most dear as collateral." Kouga lowers his gaze... and we fade in to Kaoru painting and happily oblivious the cards floating behind her, waiting to take her out. YMMV, it could also be a case of What the Hell, Hero? since he's essentially using his girl as collateral.
Kaoru and Gonza's good natured teasing of Kouga when Zaruba tells them that it was Kouga's idea to give the little flying fish the name "Kaoru".
From Beast of the White Night: Practically every scene with Kouga and his parents is heartwarming, but especially the end, when the spirits of his parents linger to bid him farewell, and he bows formally to them. They don't say anything, but it's obvious they love him and are so proud of him.
And then Kouga going home to find Kaoru at their house, waiting for him.
Kouga's flashback to the training camp, and how he and the other three boys become friends and teammates.
At the end of GARO and the Wailing Dragon, Kouga and Kaoru reuniting.