3.971 out of 5 from 819 votes
Shirihama Kenichi is a weak scaredy-cat, and his status as the karate club's punching bag has earned him the name of ‘Weak Legs'. One day Kenichi meets Miu and, impressed by her skill after she effortlessly throws him to the ground, he follows her to the Ryouzanpaku dojo. There he finds five great masters of martial arts, and driven by his desire to become stronger, he begs them to accept him as their student - though it won't be easy. Kenichi endures harsh training (and injury) from Akisame - the philosophizing Jujitsu master; Apachai - the Muay Thai ‘God of Death'; Kensei - the ecchi Chinese Kempo Master; and Shigure - the weapons prodigy. Finally after proving his determination, he is accepted by Shio - the 100-dan Karate Master, who had initially wanted nothing to do with him. Now Kenichi walks the long and arduous path to becoming a master himself, all whilst trying to protect those he cares about from anyone who would dare to harm them.
Find Kosukegawa a little weak in the knees? Check out History's Strongest Disciple Kenichi. Miu, with her crazy surrogate fathers and delicious curves could be Makoto's cousin, but HSDK focuses on the arc of the male protagonist and contains as much over-the-top martial arts action as Change 123.
History's Strongest Disciple Kenichi and Change 123 have similar plot elements. Both begin with a weak boy befrending (and falling in love with) a bespectacled girl who kicks ass at martial arts. At this point those series do resemble each other quite a bit, but they take two different turns afterwards. In Change 123 the boy stays weak, but there is a significant ammount of romance. On the other hand in History's Strongest Disciple Kenichi the boy gradually becomes stronger and starts to kick ass himself, but the romance isn't really progressing. If you liked one of those series (at least partially), you might want to check the other one as well. Just keep in mind that Change 123 contains quite a bit of ecchi (despite that the story is still there).
Change 123 and Rappi Rangai at 1st glance can be seen as totally different but in fact are very alike. The genres are both ecchi and the artists are not trying to hide the fact. The two plots are both about 3 woman trying to protect a super weak boy. One difference to note is in Rappi the 3rd woman is a man cross dresser, and in Change 123, the 3 women are all 1 woman.
Besides characters, these two manga revolve around martial arts. Every character (besides the two weak boys) knows some top notch martial arts technique that enables her to annihilate any competition. So if your interested in martial arts and awesome ecchi manga, by all means get reading these two manga.
If you liked how Kosukegawa from Change 123 was constantly saved by his lovely ladies, you might want to give Rappi Rangai a try. In that manga the male leads is protected by a harem of ass-kicking ninja girls. What's more, the protagonist is more likable than the one from Change 123 and actually becomes stronger as the story progresses. Rappi Rangai does have it's own share of problems though. The story oscillates between good and so-so during the first half and goes really downhill in the second one. On top of that the romance isn't that great. I mean, there isn't much progress with the one girl that actually matters through most of the story. At least there is less ecchi than in Change 123.
Both series are about a girl with multiple personalities. In each case, her normal personality is calm & polite personality, but sometimes her hidden identity comes out and she becomes very violent & very deadly.i
I could go on about the similarities between Change 123 and Sekirei such as girls in skimpy outfits fighting and kicking ass at martial arts or weak male lead being protected by said girls all the time, but my real reason for this recommendation is just one: both of those manga feature a male lead who harbors feelings for every girl in his harem. It might sound like nothing special, but for me it looked like a rare theme among the few harem series I've read.
P.S. Both of those series have interesting stories despite also having large quantities of ecchi.