Hmm.... okay...
I am somewhat incline to believe that it could be 'G' for Ginny but then if look at it closely... the rounded character is placed exactly under the sharp edge of the heart... and I thought, if a person could even care to carve his name and his 'partner' on the bark of the tree, wouldn't he at least make sure it aligned properly... meaning that, the rounded character might not be the first letter of a name so to say...
If it's G for Ginny... it'll be out of centered aligned symbol... so... nah...

Hermione could be a good choice... for the O in her name...

Oh, hold it.... maybe I could produce examples as well... hold on now...
Ah, right! Here there are... results of a few minutes playing with Photoshop...
Harry *heart* Hermione - possible... hmm...
Harry *heart* Ginny - unless the carver is not good with aligning... (at all!!

)
Harry *heart* Cho - with C as the rounded character... (maybe this carving is made before OotP? Who knows... )
Harry *heart* Cho - with O as the rounded character...
Harry *heart* (guess who?!!) - heh heh heh... couldn't resist!! Especially when it's the most suited among all!!

Thought I see something like this one of the comic relief books... the one with 'Quidditch Through the Ages' I think...
And.... **drum rolls**
Harry *heart* 
- okay... don't all hurl their monitors at me all at once now...

Heh heh heh... **scrurries off and hides**