Check Up by Kanan, since when he is giving her her check up and tells her to undress (no, not like that, you sickos, knock it off), she was just like, "Okay! :3" xD)

As for evidence, I'll start with Piyo Piyo Piyoko-chan since that particular doujinshi seems to be focused on these two. In one of the last stories, Kyuara mentions cooking food for one's "suki na hito", or "liked person". I suppose that would make more sense translated as "crush" in this context (since she had the little sparkly hearts going and everything :P). After this, Piyoko starts pondering over who her "suki na hito" might be. She even thinks about Coo briefly, but after he does yet another thing to make her mad (this seems to be a theme in the manga, heh), she decides she just hates him. xD Although she alternates between "hating" and liking Coo a lot, so I wouldn't take that seriously.

I'm rambling. *ahem* Now this next part, I'm not sure where the page is from, but I'm 100% sure it's still by Hina (I really had to squint to see those darn kanji symbols...). It's not in my volume of Piyo Piyo Piyoko-chan, but there's a "1" on PPPC's cover, and I have some random chapters from the second volume, but I can't find the second volume for purchase... not available yet? Anyway, Kyuara is asking Coo who his "suki na hito" is, and the first thing to mind is none other than Piyoko. I don't have the next page so I don't know what he says (=_=) but if that isn't some kind of valid evidence, I don't know what is.

You know the stereo-type that young boys pick on the girls they like? I believe Hina is utelizing this stereo-type in PPPC. Coo seems to be constantly picking on/annoying/pushing around Piyoko and deliberately being much nicer to Kyuara to see how Piyoko reacts. That might just be my take on it, but that's what I think (especially since we all know Hina is a shipper anyway xD)

Piyoko, later on in the PPPC, outright confesses to Kyuara that Coo is her "daisuki na hito" (daisuki = like a lot/love). I'm sorry, but that's pretty obvious. XD At this point, Kyuara seems to back off (SHE KNOWS SHE CAN'T COMPETE WITH PIYOKO'S MOENESS... -gets hurt in some way-)

This is a brief point, but Ky's obvious jealousy over the fact that Coo is much closer to Piyoko than anyone else reaffirms the fact that the two share an intimate bond simply by the fact that Ky is jealous.

Ky is not the only one who acknowledges Piyoko and Coo's relationship... Rik does as well. And he's not afraid to tease Coo mercilessly about it either, as seen in Fair Weather Laundry (by Yuzuru Asahina). He's very blatant about it, too, saying, "Surely, the one who troubles to lay the lady's stockings right over left is he who cares most deeply about her. Yes? *sly smirk*" Coo only blushes and denies it throughout the course of the story (a very boyish thing of him to do, of course).

This next one doesn't need too much explanation... episode six. I am, of course, referring to "Coo is a Doctor Pyo!" (from the Leave it to Piyoko anime series) Adapting (VERY loosely) from Hina's story, Coo vs Piyoko, there is a flashback to Planet Analogue, showing a much younger Piyoko and Coo. Granted, the anime episode is very different and much goofier than the manga (I really wish they'd take the characters more seriously, if only once in a while... sheesh), but still equal in fluff.

I don't want to spoil it for those who have yet to see it, but it retells the story of how Piyoko gave her panda to Coo (this wasn't mentioned in the manga version of this story) and why Coo became a doctor (which Hina touched on a bit in Why I Became a Doctor). Coo left (at a very young age, obviously, since this is even younger than his current 13-year old age...) to become a doctor so he could do something useful for Piyoko. There is a lot of teeth-rotting-cuteness interaction between the two here, such as the first time Piyoko actually cries in the anime (Piyoko is notorious for crying in the manga, but never does so for real in the anime).

In the scene (which is a flashback within the flashback, to even further back... wow XD) where Coo has a fever and Piyoko comes to comfort him with her panda, he says "Thanks, I feel better. I like you a lot" (but this last line is only in the manga). Piyoko later on in the story (in current day time) offers him a box of biscuits with "a panda mascot inside" to make him feel better, as he has a cold. In bed, even later on in the day, Coo realizes Piyoko had misinterpreted his "I like you a lot" as being for the panda, not her. He actually cries a little (which is another first!) and says, "It's very like her... silly... silly girl..."

I want to see the manga of this scene in Japanese. :| The only thing I can think of that would translate as "I like you a lot" is "daisuki", which could also be read as "I love you". *swoon* >_>;

This is a small thing, but in a commentary, Koge-Donbo (the original creator, lest I need remind you?) and some other women agree that Piyoko and Coo are probably the only real pairing in Di Gi Charat. Score! xD

New! This is sorta silly, but I'd like to mention it. xD Japanese people have strong superstitions about blood types affecting one's personality and compatibility with others, so I looked up the compatible blood types. Coo's bloodtype, O, is only compatible with O and AB, and guess who has AB type blood? Piyoko~!! -laughs- AB types are compatible with everyone.

In conclusion! (Pay attention, there will be a test... *laugh* Just kidding.) I wouldn't be surprised at all if both had feelings for each other. Coo seems to have caught on to his a lot faster than Piyoko, since Piyoko is presently still a bit oblivious. With some time to mature a bit, I'm pretty sure that both would be able to honestly say they love each other.

I hope. >_> xD

Anyway, I hope that wasn't all too confusing. I think I went off topic and babbled a lot, but hopefully, you caught the gist of what it is that's so adorable about these two's interactions.

Plus, I'm a sucker for child relationships. I-it's just cute... -sigh-