Here's how Junpei's mechanic's works:
Junpei:

Strikes - 3
Balls - 4
Out - 3

Round 1:
1. Landed 1 hit, Went to first base
2. Whiffed a special, receieved a Strike.
3. Liz hit Junpei, Strike was taken away, Out was gained
4. Did a combo, went to secdond base (Still 1 out)
5. Whiffed 2 specials and recieved 2 outs.
6. Got hit, lost both strikes and recieved another out (2 outs)
7. Got hit again, gained the final Out, bases reset.
8. -Learning Junpei intermission- (He landed 3 combos and got 3 bases, landed a normal and went to level 1)
9. Whiffed a DP and got a Ball
10. Did a combo, leveled up to level 2 (Bases were still loaded from leveling up from level 1)
11. Landed 2 more hits and leveled up to level 4 (Bases still loaded)

Round 2:
1. Level 4 still, bases reset
2. Gets hit, receieves an Out.
3. Whiffed a DP, got a Strike
4. Whiffed a normal, got another strike
5. Whiffed a special, got a 3rd strike, recieved an Out (currently at 2)
6. Got thrown, everything was reset (still level 4)
7. Shit happens and can't see any more.

Round 3:
1. Still level 4, empty field
2. Got a hit, First base.
3. AoA, Second base.
4. AoA C follow up, Third base.
5. Liz blocked, recieved 1 Ball
6. Got hit by a DP, lost ball but receieved an Out
7. Got hit by Counter, recieved a second out
8. Got hit by 5B, 3rd out and empty field (Still level 4)
9. Nothing interesting happens.


Summary:
Junpei seems like a Frank West (from UMvC3).

Whenever he lands a hit with a bat, he gains a base, 4 bases = 1 level. He'll continue to gain levels until he receives 3 Outs.

Outs are achieved by getting hit.

whiffing a bat will earn a strike and 3 strikes will earn an out (3 strikes and you're out!?).

Blocking a bats grants him an Ball, 4 Balls = 1 base.

Levels carry over from round to round (Max is 10, higher level = more powerful (unsure how powerful)) but Outs / Strikes / Balls and Bases do not.

Strikes / Balls are reset when he gains a Base.

At first it seems a confusing but it's actually pretty simple, think of it as a hype meter:

Hitting things = Hype
Watching someone block your shit = kinda hype?
Getting hit = Not hype

His Super gains 2 levels automatically

Strikes and Balls are reset when a base is gained