Mega Man Battle Network Secrets and Unlockables (GameBoy Advance)

MegaMan Battle Network is the first of the MegaMan Battle Network series of video games by Capcom. The story takes place in the year 200X, though the setting is a sort of parallel universe in which key events occurred differently from the original incarnation of the title. The player alternately controls two main characters, Lan Hikari (Netto Hikari in the Japanese version) and MegaMan.EXE (Rockman EXE, also the name of the game itself).

The former human, however the latter, MegaMan, is a computer program, called a NetNavi (derived from Network Navigator) designed specifically to facilitate the user's (Lan's) interaction with the Net and other computerized devices. In the series, the internet and the inner workings of computers are displayed as a material world which computer programs of all varieties, as personified in a humanoid form, can interact with.

Unlockables

Secret Boss Navis
After beating LifeVirus, you can fight four secret Navis if you meet the requirements.

Bass(Internet Area 16) - Have all chips in Library excluding #127(LifeAura).

MagicMan(Internet Area 14) - Simply have beaten LifeVirus,

PharoMan (Internet Area 12) - Be at least at Level 70.

ShadowMan(Internet Area 15) - Have at least 140/175 Chip Library.

Secrets

Entering Deeper Parts of the Internet
As you make your way through the many areas of the Internet, you'll notice that there are gates in the way it links to following areas. Those gates have question marks on them, and you must fulfill a certain requirement within that area of the Internet, then go to the gate in order to open it and proceed to the next Internet area. Here's a quick list of requirements to get into the later areas:

Area 11: You cannot escape any battles in Area 10.

Area 12: Busting Level after every battle must be above 3 in Area 11.

Area 13: You cannot escape any battles in Area 10.

Area 14: You must win at least 10 chips in battle from enemies in Area 13.

Area 15: You must stay out of battles as much as you can in Area 14 (less than 7, I believe).

Area 16: Busting Level after every battle must be above 4 in Area 15.

Meet Forte (Bass)
Collect 174 chips, all except Dream Aura chip (No.127). Go to the deepest Internet Area (Internet Area 16, in the area where you can get Paradin Sword). Walk around that area until you find him. He can appear everywhere in that area. If you can beat him, he should give you the Dream Aura chip. Please remember, Forte CAN'T give his chip to you.

School Network Passcodes
In the School's computer network, there are several doors you'll come to that require you to input a 2-digit passcode. Here's a list of the codes for each door (for the ones marked as random, you're given hints if the number is higher or lower each time you guess wrong):

Door 1: 09

Door 2: 30

Door 3: Randomly generated

Door 4: Randomly generated

Door 5: 15

Door 6: Randomly generated

Door 7: Randomly generated (Requires Teacher's Passcode to access)

Here's a video collection of action sequences taken from this great title: