Usually sequals tend to be worse than the originals, but, Birdy the second season manages to flip that theory compeletly upside down.
The sequal focuses much more on Birdy. Tsutomu probably appeared for a full fifteen minutes (or less) if you combined all the scenes where he was in. The main focus is probably Birdy's relationship with Nataru and their pasts, and this time, there is no "saving the world" thing going on. It's on a much smaller scale when it comes to that. Of course, there still are "bad guys" although, you start having doubts on how evil they are as the series progresses.
What I really liked about this series was that action moments were interwoven with slice of life elements. They both complemented each other. Their purpose for fighting expanded their character dementions and left you on the edge of your seat. Characters were shown that had little to do with the series, but still showed how they were living on after the drastic destruction that took place in the first series.
It can get weird at points, but other than that, it is nothing short of astounding. The fight sequences, the scenery, all of it is breath taking. You should definitely watch at least a trailer of Birdy, just to see how amazing the animation is.
The soundtrack is significantly better in the second season of Birdy, even though it was also very good in the last one. I actually have the soundtrack on my iPod, that's how amazing it is. It fits very nice with the story and scenery and sets the right tone that evokes emotions more powerfully.
Birdy really comes out in this series. A lot is revealed about her and her personality, and the relationship she establishes with Nataru, is a delight to watch. Almost all of the characters you meet are memorable, and sometimes confusing (you'll see what I mean if you watch the series). There are twists on the characters personalities that lead you to second guess them.
Watch it. It's another one of my favorite series, and I'll probably go back and watch it in the future. It's series like these that don't make me lose faith in anime. It gives me hope that there are still gems amidst the over abundant modern crap, wating to be found.