Sakura is Kero-chan's closest friend. They seem to have both a brother-sister and a parent-child relationship, and somehow their parent-child relationship works both ways…
Kero-chan spends most of his time with Sakura, and although they get along well, they have their fights. They usually involve one of the two acting childish, Kero-chan eating something he shouldn't have, or Sakura being unable to save/bring Kero-chan dessert.
Their friendship faces its most difficult test during episode 15, when Kero-chan builds himself a Kero-chan-sized room in one of Sakura's desk drawers, complete with little furniture and wall decorations (sticker pictures of himself, of course). When Sakura finds out that Kero-chan has dumped all her stuff on the floor to make room, she angrily throws Kero-chan and all his little furniture out of the drawer.
Kero-chan gets very upset by this, so when he later finds a box of chocolate he assumes belongs to Sakura, he doesn't hesitate to eat it all. But he doesn't know the chocolate is filled with brandy, so he ends up drunk in a dingy alley, talking to a large toy crocodile about his problems. To make matters worse, the next morning a little girl finds him and takes him home thinking he's a cat (^_^;). When Sakura realizes he's missing, she gets extremely worried about Kero-chan and has some time to realize that he just wanted a little privacy and personal space. So when they're finally reunited, Sakura welcomes Kero-chan home with a new room, and all is well again (at least until Kero-chan accidentally breaks the bed Sakura made for him by jumping happily on it).
In spite of their occasional quarrels, Sakura and Kero-chan share many more happy times than bad, and they're very fond of each other. Sakura looks to Kero-chan for guidance, brings him out with her whenever she can, and always does her best to bring him dessert. In turn, Kero-chan guides Sakura, protects her as best he can, and has great faith and pride in her as his chosen Cardcaptor. They make a great little team, and when Kero-chan finally gains the ability to return to his true form, he protects Sakura with his full powers.
Kero-chan's desire to protect Sakura is probably most prominently shown in episode 6, when Sakura gets hurt emotionally and physically by The Illusion card. The next morning, Kero-chan brings her a flower while looking very sad and tells her he doesn't want her to be hurt or sad just because he chose her as the Cardcaptor. Sakura is really moved by this and gives him a big hug. ^_^
In my opinion, Sakura and Kero-chan really think of each other like family. They count on each other being there, and I don't think they could easily get used to being separated.
