Post characters who immediately/instantly o almost immediately/instantly become best/close and/or even lifelong companions, partners, pals, etc. o all of the above

Shinpachi and Kagura immediately became close with Gintoki right after they met, its sort of más like a family relationship than the ones tu dicho so I hope this counts

after living their daily lives alongside the natural permed idiot they became más and más like a family..... a completely whacked out family

Natsu, Happy and Lucy (Fairy Tail) become immediately friends and partners after Lucy joins Fairy Tail.

sket dance members...bossun,switch and himeko!

Ginji Amano and Ban Mido from GetBackers. It wasn't the MOMENT they met, but close enough!~

naruto And Sasuke Were Instant friends And Rivals :3

Haru&Elie from Rave Master , even though the way the meet was "below the waist", haha, they still became very close friends almost immediately.