Honestly Saejima doesn't get much of any development in Yakuza 5. His section feels sorta the most shoehorned in and he definitely doesn't get much freedom, not to get too spoilery.
My guess is because he has a different role in this one; In 4 he was sort of The Final Character, so he was meant to come across as more powerful, while he's the Intro Character in Yakuza 5, so he's pretty much the one meant to ease you into the combat system.
That's one of the flaws with the Multiple Character system, though I think 5 improved on it in many ways over 4.
That is a real shame to be honest, as I am pretty much a fan of Saejima's shenanigans from 4. (I actually captioned what he says on the final Side Quest on 4 because I loved it so much!) Appearance aside, I do like how he comes across, but if what you said is true, poor Saejima....surely SEGA (or Nagioshi) could have thought of SOMETHING!?! He's a convicted criminal with a heart of gold! (although they all are to be hair), having him learn the ropes again in the Tojo Clan would have been nice to see, and perhaps show how he advances in the modern world too.
Does Akiyama at least develop more in this game? I liked his story about being a legit money lender but I think it got boring after awhile.
Regarding the latter, I did come into the game expecting to start from scratch, but after getting to level 15/16....meh....he doesn't feel the same (guessing this is related to the story about him being rusty).
Random question but....is it me or does Kazuma look YOUNGER every game he appears in? (Appearance wise, he looks younger in 6 then 5, and then in 5 from 4 due to how they make him skinnier).
@Aki: Is the Monster anime by any chance about [[spoiler]a Surgeon doing an operation on a kid and that kid starts killing?[/spoiler]
Yakuza and Anime kind of makes me think of Boondocks....