Sagara Sousuke is far from the average high school student -- not only is he highly trained in military tactics, he puts his knowledge to use in every single applicable situation (and even those nonapplicable ones.) So, when confronted with high school, he tends to turn average high school experiences into off-the-wall adventures. Love letters become terrorist threats and field trips become commando operations; his survival depends on the watchful eye of his best friend and classmate, Chidori. Together, they may just make it out of high school alive...
Haré and his mother live peacefully in the jungle, until one day the boy is overtaken by a omnipotent shadow. Later, he awakens to find Guu, a strange girl with even stranger abilities - notably, the ability to switch from a cute, lovable Guu, to a menacing delinquent who eats everything. Now Haré must live with Guu, and lead a normal life, despite her oddities.
At the heart of both of these series, they're about a boy and a girl--one of whom that is a fish-out-of-water and causes chaos wherever they go, and the stressed out, hot-tempered "ordinary" person that gets dragged into that chaos kicking and screaming.
There's definitely some differences, of course. Sousuke, poor guy, TRIES to fit in but just can't manage it due to his background, whereas Guu obviously knows exactly what she's doing and is basically a massive troll. Also, Fumoffu is much more focused on the romantic aspects of Sousuke and Kaname's relationship, and Haré+Guu really only brings up the idea of the two mains being paired up to make a joke every now and then. Fumoffu is also a bit more grounded in reality (or at least anime reality) while Haré+Guu is set in a bizarre, cartoony world.
Still, since the key focus of both series is the relationship between the main leads, and their interactions come off as similar, I think, so I'm still fairly confident that if you're a fan of one you'll find the other equally hilarious. Also of note, if you're a fan of Robert in Haré+Guu you'll love Sousuke, and vice-versa.
(As a side note, Fumoffu is technically a sequel to the original Full Metal Panic, which is equal parts romantic comedy and more serious mech action-drama. You can probably still enjoy Fumoffu even if you haven't seen the original FMP!, as its really not too related to the plot of the original, but if you're interested in Fumoffu and the original FMP! also sounds interesting, start with the first season. If you just want the comedy, though, try out Fumoffu.)
Drawn to the mystique of the unknown, Hunters travel the world in search of terrifying creatures, incredible riches, and unexplored lands. Gon Freecss is a naive-yet-determined young boy who aspires to join the ranks of these individuals, in order to find his missing father Ging - a master of the profession himself. To reach his goal, he partakes in the formidable Hunter Exam, a series of tests that push the participants to their physical and mental limits, with a Hunter License as the prize. During the exam Gon befriends vengeful Kurapika, doctor-to-be Leorio, and skilled assassin Killua, who have entered for their own reasons. But with the sinister Hisoka standing in their way, will Gon and his friends be able to succeed in obtaining their reward, or even escaping with their lives?
Kotoko Aihara has always been in love with Naoki Irie; he’s tall, handsome, and rumored to be the most intelligent student in Japan – who wouldn’t be? Only one thing stands in the way of a relationship with Irie: Kotoko is ditzy and is at the bottom set of her school, and Irie-kun hates nothing more than ditzy, stupid girls. When disaster strikes and Kotoko is forced to move in with Irie-kun’s family, she suddenly sees her chance to bridge the gap. After all, even cool, super-intelligent guys must have a soft spot, right?
Now i know what your thinking what is the connection between these two anime. Well is both one of the main love interests is initially cold. Kaname gets easily annoyed with Sousuke but she starts to develop feelings for him and the same is true for Itazura na kiss. In both of these the romance is really slow but your waiting for them to tell each other and so you don't mind the wait.
Years after having been exiled from the Kannagi household for being unable to utilize their fire magic, Kazuma returns to Japan, only to discover that the Kannagi family's very existence is threatened. Unknown to them, during his absence, he had become a Contractor and thus acquired a vast strength in wind magic. Not showing any remaining contempt for them, he offers to help solve the Kannagi household’s problems in return for large sums of money. Together with his younger brother Ren and the feisty heir to the Kannagi household, Ayano, he fights to protect those he cares about... for a price!
Galan is a failure of a samurai who is down on his luck, until one day, the unexpected happens: a princess of a foreign country presents him with a sword. To his delight, he accepts it, completely unaware of its past and its secrets. Shortly after, strange events begin to unfold, causing both humorous and ecchi situations for Galan to survive through, all the while protecting his mysterious sword full of secrets.