Reboots get a bad rep, and not without reason. Often they're associated with lazy cash-ins, over-thought redesigns, a lack of originality, or some combination of the three. Birdy The Mighty: Decode is a reboot of sorts. More properly, it's an adaptation of Birdy The Mighty II, itself a reboot of the original Birdy The Mighty manga. Thus most properly, Decode is a reboot that is also an adaptation of a reboot. Quite confusing! Somehow though, through all these changes of hands, Decode manages to avoid most of the pitfalls associated with franchise rebooting, and forge a strong story all its own.
The original Birdy The Mighty OVA was a pretty succinct tale. Having no room for extraneous storytelling meant that it was punchy (often literally), to-the-point, and direct, creating a simple, fun romp more or less devoid of deeper themes but undeniably excelling at what it set out to do. Decode is a much larger story, and right from the beginning far more wrinkles are introduced than in the original. In contrast to the OVAs where each character was in all respects--motive especially--quite simple, Decode has more room to be spacious, and its characters have more angles. For one thing there are simply more of them--as many characters are introduced in the first episode as are introduced throughout the OVA's entire run--and for another the story has more space to give them each a bit more going on. In the original OVA we don't learn very much at all about Birdy's time before she came to Earth (aside from some brief flashbacks which were never elaborated on). In Decode her pursuit of an alien criminal is the prelude, and by the time she finally encounters Tsutomu at the end of the first episode she's already been on the planet for several months and has adopted a cover identity as an idol/model while searching for the alien superweapon (later revealed to be a biological parasite) known as the Ryunka.
But Decode isn't content to simply differ in terms of scale. Around the end of the third episode it becomes pretty clear that this series aims to have stakes, and the early death of Birdy's assistant (the octopus-like Tuto) is a fairly sharp jab, although I suspect whether it truly connects will depend on how tied you--the viewer--were to the character. The fourth episode, in a fairly sharp turn, is effectively a miniature court drama, and it's here where Decode leaves the self-contained world and thorough action focus of its predecessor behind for good.
The effect of all this is that Decode is in its first half especially, a bit all over the place. Decode's cast balloons from substantial in the first episode to borderline excessive by the series' halfway point, with so many characters (many of whom aren't terribly visually distinct) it can be hard to keep track of--much less care about--all of them, and some clumps, particularly Tsutomu's male classmates, and Birdy's fellow space cops, kind of blend together into indistinct blobs. Others stick out more, especially Keisuke, a down-on-his luck, conspiracy-obsessed reporter, Tsutomu's classmate and love interest the waifish Sayaka, Birdy's enigmatic nominal compatriot Nechla, her mentor the dinosaur-esque Scherzo, and several compelling villains, such as Kinzel Hower (a jackal-headed terrorist who makes a big impression despite only being in a small part of the series), and chiefly, Satyajit Shyamalan, an Indian-American businessman who pivots from rather boring in the series' first half to quite cacklingly insane in the second, and of course there's the cryptic alien G-man Gomez, one of the few characters who returns from the OVAs more or less unchanged.
In addition to the glut of characters, Decode has a habit of mixing comedy, fairly serious stabs at drama, action, a pretty shameless amount of fanservice, and from time to time, light horror. This is not inherently anything bad, many anime mix and match elements of different genres to great effect (many of the all-time greats, in fact), and while it's hard to call Decode's use of this technique "bad", it's not exactly stellar either. It often feels like a lot happens in single episodes, and the large cast means that many plot threads are often running at the same time, which taken all together can make individual episodes sort of run together unless they focus on a smaller subset of those elements, and without fail, the best do.
In particular the action is never anything less than stellar. From expertly-done fights (especially Birdy's confrontation with a battle droid dressed as a maid in the 9th episode, and a neat sequence in the second shown from her perspective in first-person). The drama too is surprisingly compelling at times despite a habit of leaning on the melodramatic and on odd plot contrivances in a few instances. It's hard to fault the series too much for this, given that these are problems that are more or less endemic to action anime.
As the series nears its conclusion its focus tightens, and much of the comedy is ditched in favor of a narrower focus on the action and drama. It also does get rather dark, which is probably not a surprise per se given Tuto's early death, but when the series begins adopting elements of the world story (a genre that flattens out and equates person-to-person emotional issues and broader, often world-endangering threats), particularly Tsutomu's relationship with Sayaka developing just as Birdy--without Tsutomu's knowledge--confirms that she is the Ryunka, you really understand just how far from the relatively straightforward romp of the OVAs we are in terms of tone, episode 11 in particular is downright stomach-turning.
But perhaps smartly Decode avoids going full-on Evangelion by scraping together a happy ending (near the last possible minute I might add), suffice it to say, things end decently (though not perfectly) for our protagonists, and in the end, what happens is that despite their wildly different aims and the decade between their releases, Decode ends up being oddly similar to its OVA ancestor in a pretty important way. It aims to hit a few particular points, and, despite the mixed execution early on, it ultimately hits them almost exactly. Could Decode be better? Almost certainly, with a few of its plot contrivances ironed out and its cast trimmed it could be great instead of merely quite good, but it is very hard indeed to knock a show for being "just" a compelling story from beginning to end.
Birdy is more or less a superhero, and I think that Decode is best considered in those terms. The OVA is a golden or silver age story, simple and full of fun action. Decode is more akin to a modern comic, the spirit of those old stories is kept intact, while the specifics are updated for a different audience. If you liked the OVA and don't have a total aversion to anything "dark", you'll probably find a lot to like in Decode too.