Kämpfer

Both are similar, Girl to Girl Combat (Fighting). Girls are Fighting the other team, alll Girls have different kind of abilities to Fight. It also has some Ecchi Bits which alot of you guys might like. The only Difference is that in Sekirei the girls need a Ashikabi (contractor) while in Kampfer they need to be chosen by the Moderators (more like god type person). 

Both are set during battle tournaments were super-powered girls wearing impractical outfits routinely beat each other up. Fanservice ensues. Sekirei is the more explicit of the two, but it also has a better balance between fanservice and fighting. Kampfer is mostly fanservice. 

Both have sexy girls that fight in a tournament and have special powers. 

Both animes have plenty of fan service and deal with girls with some awesome powers. If you're looking to watch some pretty girls fighting around town, then Kampfer and Sekirei are perfect. 

-Both animes are about receiving or meeting a third party to supply you with powers to fight in a game, and aim to defeat the other opposing parties to overcome it.  

They are similar because a boy get stuck in a game and does not went to fight. 

Both are Harem/Action,both are about girls(and few guys)who are put into a survival game without even knowing why they must fight.In both the main charcther is pretty usless so they have to rely on the girls around them. 

Girls on opposite teams fighting each other using special powers. Both are ecchi series, but Sekirei has more fanservice. 

In both shows, the character transforms into a girl in order to fight. Kampfer isn't as action oriented, mostly comedy and Ore, Twintail ni Narimasu is both action and comedy oriented. Both are, what have you, "magical girl" anime, in a sense. The only major difference would be that Natsuru doesn't like transforming into a girl while Soji...well, he does because he likes twintails. 

Transforming from boy into a girl is what connects these anime the most. Senou Natsuru from Kämpfer have to do it against his will, but for Mitsuka Souji from Ore, Twintails ni Narimasu. it's just a side-effect of using battle-suit. Both shows have elements of mahō-shōjo, romance, comedy and ecchi, and Gonna be the Twin-Tails!! has also some elements of mecha. If you liked one of those, it's very likely you would like the other. 

* In both shows, tha main male character transforms into a girl. * The main character have a childhood friend and the Student council president joins to their team. * Both have some fanservice. (Twintail less than Kampfer) * Similar genres 

Both are amazing comedies with a gender-bent main character, who has to learn to deal with living as a girl. Kampfer is more full-on with this, as the main takes a while to learn to control the transformations, while TailRed mostly controls his transformations... 

They're both about boy turning into a girl and fighting villains. Twin tail the boy is more self sustain, unlike Kampfer. 

-Both animes are about receiving or meeting a third party to supply you with powers to fight in a game, and aim to defeat the other opposing parties to overcome it.  

Honestly the ridiculousness, unpredictability, and the genuine uniqueness of these two shows is uncanny and I'm not even trying to make a pun. I loved them because they just kept me guessing and laughing the entire way through. They probably won't sell you on plot, but I definitely think the selling point for these shows are how much fun you have watching them.  

Fighting isn't easy especially if it's for a prize and in both of these series, competition is what strives the characters to fight. Both have a lot of drama, comedy, and later romance added into the mix. And of course, there is action to determine fate. Both series also contain a little yuri (hehe)  

these both involve two different types of girls who are required to fight to see who is ultamitely the best 

- MSO (MC is Girl), K (MC is boy) - MC get powers only while transformed in his opposite gender. - Love interest of MC only likes his/her transformed form. - Both shows are hilarious 

- gender-bender comedy anime (in Kämpfer it's male -> female, in MSO it's female -> male) - parody on the mahou shoujo concept - the crush of the main loves their second (K - female, MSO - male) form 

Both are fighting animes with genderbend elements, although Kampfer is more ecchi when compared to the absurd comedy of MSO. They both have fanservice too, although in different ways.  

Same scenario only reversed  

A boy is turned to a girl and otherwise. 

Both are about boys that sometimes turn into girls, both are very funny as well. If you like a good long story with something diffrent I think you schould cheek out Kämpfer and Ranma 1/2. Lesbian, Gay, Bisexual, Transgender will love this as much as straight people but I can see how an LGBT person would like this one alot!  

These two shows are 50% similar to each other I guess because they both go their separate paths throughout their storylines. 1. They are both gender bender shows 2. They both have plenty of nude scenes 3. They both contain fighting scenes (for all of you action lovers) 4.They both have catchy credits 

About a boy who is also shoved into the world of the females and has to deal with awkward situations as he tries to fit in as a girl one day and a boy the next really funny a must see 

The main male character used to have a normal life, until it turns upside down when he participate in a fighting combatant on a female demographic area. At first it was embarrassing and difficult to adapt the environment, he soon get used to it and play along the rules. There are funny/cute moments, the main male character can be at dense sometimes, though they have some good action scenes. Kampfer follows a male character who turned female when he was contracted as a Kampfer. Infinite Stratos, follows the character who studies at school filled with an all female cast, using IS as a fighting combat. 

Both are harem. The main guy(who somehow/somewhat ends up looking like a girl) transfers to a school, which is for girls only(in Kampfer's case, it's the other SIDE of the school). They act a bit girly, but Mizuho is somehow more manly(in my opinion). They are also very beautiful in their female forms(even more than their real male-self), and is popular in school. Kampfer is more inappropiate, though. 

Both have a little bit of shoujo ai in it. However, I think Otoboku has a better plot, and Kampfer is more of a fanservice. 

Un amour impossible, de l'action, de l'humour c'est deux animes on le même font même si sans aucun doute Tetsuwan Birdy Decode et plus approfondir que Kämpfer en tous cas si vous aimez l'un vous aimerais l'autre ! 

Same genderblender. Story is about a boy that gets killed then he has to share a body with the girl till his body has been fully repaired. 

Both stories are about a guy who likes a girl in their school and for some reason or another gain some mysterious power which they dont want, to transform in a girl for kampfer and talking to cats in Nyan Koi! Throughout the story both also share similar circumstances where girls around him begin to fall for him. Both animes are also fairly new. Both also have some fanservice at times but not too much to spoil the anime. Though the girls in Kampfer are much more aggressive than those in Nyan Koi!  

Both the series are Harem, with good lucking female characters who tries their luck with the main character sophisticated teenaged boy.. 

Both shows are about a guy who by some alien force is transformed into a female, however each show takes a different approach to this. Kämpfer takes an ecchi route while Kashimashi: Girl Meets Girl takes a more serious approach. Both shows have a romance plot but each takes a different approach to this. Kämpfer takes a crazy Harem and takes it to a new level while Girl Meets Girl takes a more serious approach again to its romance plot. 

-Both animes are about receiving or meeting a third party to supply you with powers to fight in a game, and aim to defeat the other opposing parties to overcome it.