I looking for more of this type manga. Pretty much, shy Yankee girlfriend and normal/weak boyfriend that hide their relationship. I am doubtful more exists but here hoping!

I would probably classify the female leads as slightly "tsundere" over "shy?" In which case, there's a few other comedies that may be worth looking at.

Gokusen has some simarities, a strong female yakuza member who gets easily flustered when it comes to romance. Now, romance isn't necessarily the focus of this manga, but it's definitely there; and while the boy isn't exactly weak, he's a hundred times weaker than she is.
Seishun Otome Banchou! is a relatively new manga, but it seems to be about a delinquent girl and her normal, friendly crush.

Aaaaaand lets list a few more that fit the feeling but aren't as close in content.
Samurai High School always struck me as having a simar vibe to Onidere. Somewhat about siblings more than romance, but I'll throw it out there so you can make your own call.
Waratte! Sotomura-san is about a shy girl. Definitely adorable, hardly any romance (though her classmate does make some cute attempts).

Last edited by Snakering at 9:35 pm, Jan 5 2015

I have the perfect one for you, though I hesitate to say it's in the same veins as those two you've mentioned, which are/were also in my reading list, due to the fact that this series is more plot-driven than say, Yandere Kanojo. That said, Himedol!! and its original concept one-shot, Houkago Idol are what I'm talking about.

The story is about a guy who went to this huge highschool, where in addition to the standard curriculum, there were also specialized courses for the students for whatever they wanted as their future profession. On his first day, he ended up sitting next to a yankii girl, who everyone feared. However, when he got lost on the school grounds later in the day, he encountered a beautiful girl who was singing by herself. He later found out that she was the yankii girl who sat next to him in class.

When the girl heard that he was trying to become a expert hair-stylist, she told him that she was striving to be a idol. As a young girl, it has been her dream to become an idol. But because of reasons, she was brought up as a boy, and went through martial arts training. Even though she was a physically strong yankii, she was insecure about her abilities and feminine charm. Ultimately, she decided to enrolled for the idol classes at the school, promising herself that if she fails, she would give up her dreams.

But because she didn't know the first thing about styling her hair or makeup, she asked him to be her personal hair-stylist and help make her look like an idol. The guy reluctantly agreed after an incident, and since the girl needed to keep her yankii appearance intact, they kept their relationship a secret in public, and essentially, the girl started living a double life in school as both a yankii during normal classes, and an idol-in-training during the idol classes. And since she looked completely different in both circumstances, nobody suspected a thing.

Please note that it's a pretty short series, though. =< Good if you don't like it much. Bad if you enjoy it, like me.

Last edited by DarkDusk at 10:01 pm, Jan 5 2015

Maybe The Kidnapping of Minja Jo's Boyfriend : the female lead is tsundere, not really shy.

And here are other mangas with a delinquent female lead/normal male lead, although they don't always fit your shy/secret relationship requirements :
Akuma to Love Song
Meguraian no Kimi
Show Me the Money
Shinshi Doumei Cross
Sakurasaku Syndrome