Detective Conan:A smart and conceited boy, who is also a sherlock fanatic claims to be a detective. He is dumb enough to follow some shady gangsters, which results in him getting poisoned. Defying all kind of biological rules, the poison shrinks him so that he doesn't die, but gets stuck in the body, that looks 10 years younger. Unsure what do do next he goes to his friend/neighbour, a strange old professor, who mostly invents things that just explode or are useless, which is the reason why he is neither rich nor married. With the help of his friend the boy manages to build a fake identity and to convince his 'girlfriend' to take him in. In over 600 episodes the boy doesn't manage to convey his love to his girlfriend, neither does she, while she isn't smart enough to figur out, that her childhood friend is living with her either. Her father is a useless detective and spends most of his time either drinking, playing majong or seeing horse racing. He always suspects the wrong people and is unable to chase or protect someone. Anyone who invites him to a party is crazy, because the party always ends up with at least one person dying if he is there. The boy narcotizes the old detective in order to solve cases with his voice (with an invention of the professor he is able to reproduce almost every kind of voice, even people he meets for the first time). About 90% of those cases involve either a string, a closed room or an 'impossible' crime and about 70% of the murderer are handsome men. The old detective gets more and more famous, while the boy has to go to elementary school again, were he plays little detective games with his classmates/friends. Later more and more people show up, like a little girl, claiming to be a 81-year old hag (could also be 18, but you can't be sure). Together they try to catch the gangsters in black, that poisoned the boy, who have names, that make them sound like a group of anonymous alcoholics.
Sorry for the long text, hope there aren't too many mistakes, feel free to edit^^Funny AMV fitting to the text@Siren~There are also romance manga/anime with really cool/likeable main characters, but you have to search for them
You could try Denpa Kyoushi (reverse harem), the main guy is nice and funny and the girls all have their strong parts to (there isn't really any romance yet).
Love so Life (slow romance, and the girl is really likeable)
Skip Beat (the main girl wants to get revenge on her ex-boyfriend for tricking her)
Hajimari no Niina (a girl gets reincarneted and meets the boyfriend of her former self)
I am a demon, she is a wife (girl makes a contract with a demon so that he has to marry her, she loves him, but he claims to only like beautiful woman)
Shinigamihime no Saikon (the 'The Princess of Death' marries a 'Tyrant' - very funny)
Keishichou Tokuhanka 007 (a girl with special powers joins a detective agency - very likeable main characters)
Cubism Love (the main girl had in accident and is just a brain in a box now, but falls in love)
Betrayal knows my Name (Yuki is besides Shiharu from Love so Life, the most kind-hearted main character I know, which makes the main pairing very likeable)