I must've gotten the extra circle by not noticing the transition to air battle happening before the third hit came out. I believe I stumbled onto that combo by accidentally putting in one of the other character's (V-mon's?) combos in while playing Agumon, not realizing he had this. Hidden combos are cool--if every character can secretly do what Agumon can, it levels the playing field and makes knowing your main more important. I'll test it out when I have more time to play, school's very demanding at the moment.
Unlocked Omegamon, Shoutmon DX, Wormmon, Impmon, Shoutmon X5B and am currently on Dukemon's doorstep. So far me and my friends have noticed a general trend in gameplay, of (on the surface at least) strict character tiers. While any character can make use of long combo strings and projectile camping until the opponent either attack breaks or guards them, some are flat out better at it because of priority moves, superior attack speeds, walking speeds and the relative SP costs of each character. For example, Guilmon has an awful matchup with Shoutmon because of Shoutmon's long reach and fast combos that can come out faster than Guilmon can bring out his own, which leaves White Wings (XO) as his primary offensive lead into knocking Shoutmon down and from there going into a ▢ string to try and trap Shoutmon until he can build meter for evolution to Dukemon. Trouble being that if Shoutmon attack breaks him, he has an immediate lead into his own combos with Fiery Fastball--Guilmon has no such quick approach because Pyrosphere has a long start-up animation instead of coming out instantly like FF.
In general the characters are defined around the quality of their fireballs, walking speeds and combo strings. I've found that Shoutmon, Agumon and Impmon are the best fireball characters. Each of them has a fast long-range fireball with knockback that has a low SP cost. Impmon's is the best because it has long-term homing and will chase the opponent across ~60% of a stage's horizontal range before expiring, tracking them and having two angles of attack. It can also be fired while moving, and Impmon moves quickly. Guilmon's could be good if it didn't have the long startup, because even though it costs 100 SP like Dorulumon's, Guilmon actually has the means to rapidly gain back the meter 100~200 SP at a time from White Wings hitting. He's not the worst though; Stingmon can't hit things that are standing right in front of him with his, they have to be at a distance to connect. By contrast, Wormmon is more interesting because his fireball comes out fast but lingers for several seconds on the stage, not moving through a lot of space and instead occupying it like a cloud and trapping the opponent in a stun while dealing small amounts of damage that don't grant mercy invincibility. So while with the others you're throwing it out to knock the opponent down and hopefully start a combo string right when they get back up, with Wormmon the fireball is a tangible lead into an actual unblockable string right away. As long as Needle Thread connects, your combo will go off.
I have a big issue with how the game handles its guard system; you don't build any meter off of guarding successfully. Neither the evolution meter nor the SP meter fill up when you guard, which means that playing competitively comes down to exhausting the opponent's defensive options as quickly as possible, and it's a pretty swift process. You don't get back SP when you respawn, so once their limited number of attack breaks have been exhausted it's a matter of getting the opponent into a corner with a semi-infinite combo string leading in from a weak attack into a fierce. (ie the ▢▢OO we were talking about) Some characters like Shoutmon seem to have strings they can land right out of a successful guard, but not all of them. Guilmon is SOL if his SP is empty, so for him the only option is to take the lead early in the match and maintain it constantly.
Some preliminary ideas about valid stages for tournament play;
-Factory Town, Coela Beach, King's Castle, Digital Station and Lava Pit are out because each of these possesses some form of stage hazard that can be camped to lock a losing player into an even worse situation. Lava Pit is the most subtle, its hazard drains the HP of any character standing in the outer part of the arena when it's up, while Factory Town's doesn't do direct damage but has a hole near the bottom of the stage that will teleport any character that falls through it to the top of the stage and dizzy them, leaving them open to throws, charge attacks etc. The Locomon does the same thing but with damage added on.
-Digital Plains, Native Forest, Toy Town, Tilly Valley, Proton Shrine lack hazards and each provide some kind of varied geography. Proton Shrine is this game's equivalent to Final Destination, a completely flat, featureless arena with no geographic variation. Digital Plains has different elevations for the upper and lower sections, Native Forest has more dramatic elevations plus a bottleneck at the bottom of the stage that favors fireball characters (not necessarily a bad thing when you have a loser's pick format for best of 3), Tilly Valley is similar but with the bottleneck near the top, while Toy Town is a circular incarnation of Proton Shrine with an elevated center platform. Toy Town also has the only destructible environment that I know of.