Are you serious?! Lets begin with my alltime favorite character ever. This is going to be fun. 
1. Brendan Filone is one of the 2 main supporting characters of season one, the other being Mikey Palmice. How is this guaranteed? Simple. If you check the cast listing and first credited cast, you will see that Brendan and Mikey are the only two names listed in the main cast for season one.
2. Second, simple. The newly released collector card set. Brendan and Mikey are also 2 of the only people who got killed to get their own cards. By this i am referring to the new set released by inkworks. check it out if you haven't already.
3. His death plays a prominent role in the series. he is mentioned in 6 episodes after his demise, not including "Meadowlands", where his body is found.
4. He appears in 4 episodes, not 2. And in the episodes that he appears, his name is placed prominently in the beginning of the credits.
5. He is mentioned even in season 2. Chris has a dream sequence and sees Brendan and Mikey playing cards with soldiers. Want more?
6. He is the first character in the series who actually played a ROLE and got killed. All people who were killed in season one, other than mikey, were very minor appearing. -