Mega Man Battle Network 2

After you beat Gospel, secret Navis are fightable to you, if you meet the requirements.

To unlock this special style, you must defeat all V3 navis (excluding Bass) with an S-rank. Then, the next time a style change occurs, you will get Hubstyle.
Hubstyle is a unique style that is non-elemental, and combines the benefits of every other style in the game. It will halve your max HP though.
This style will only show up once per game, so if you skip or erase it, it will be gone for good.
To confirm this unlockable, talk to the girl in ACDC Town behind the trees near the park. She will tell you which V3 navis you have defeated with an S-rank, and say that she sees all the available "ghosts" if you have defeated them all.

Put a ZeusHammer into your folder. Go into battle and take damage on purpose till you have 200HP or less. Then kill the virus or escape. Now, find a Mole2 Virus in WWW Area 3. (This can be done with Mole1, too, but it's harder.) Attack the virus till it has 200HP or less. Then use your ZeusHammer. The Mole2 is deleted and you take 200 damage. Your HP is reduced to zero but you still win the battle. Now you can walk around the net with zero HP.

Note: Have a healing subchip or you'll get deleted when next battle starts.