Code: Realize - Guardian of Rebirth is a visual novel 'otome' game, a story-driven choose-your-own-adventure style game, where the aim of the game is to cultivate a romantic relationship with one of several eligible guys. Telling the tale of Cardia, a young girl whose gem for a heart means a deadly poison flows through her veins, and her search for the scientist father who knows the reasons behind her condition (and hopefully how to control it), she joins up with five potential love interests, each of which have business with her missing father. Each with their own unique personality, back stories and endings, there's a guy for every taste - and with any luck, you may just find yourself falling in love along the way.
Like most 'visual novel' style games, Code: Realize is pretty much entirely reading, with the only real 'game' part being picking the odd conversation option to steer the story in a specific direction or grow closer to one of the guys. Played more to enjoy the twisting and turning story than for any reaction-based action, it's more akin to sitting down with a good book than playing a game in the traditional sense. Conversation options lead you down specific routes too, and without an easy way of checking how your relationship with each of the characters is progressing, if you're going for a specific ending, you'll need to think through your choices accordingly.
It's worth noting too that, while most of the game is voiced, it's entirely in Japanese with English subtitles - so a solid reading ability is a must both for understanding what's going on and for having the patience to sit through a very wordy tale. Therefore, it's probably best left to those who like, and are particularly good at, reading. Those who tend to just mash buttons to skip over cutscenes and talking bits will be skipping over the entire game if they're not careful.
In terms of mature content, Code: Realize is fairly middle of the road. Violence is depicted through still-frame cutscenes which sometimes depict characters slashed, stabbed or shot, occasionally with small amounts of blood on characters' faces and clothing. One scene shows a man with a short blade through his chest, with blood shown around his mouth and around the puncture wound.
Being a game centred around romancing eligible guys, there are a few bits and pieces that hint at sex and more risque things, although none are particularly overt. Scenes show couples kissing, or a man pinning a woman to the bed, while hints at 'naughty games' are about as explicit as it gets. In terms of bad language, words like 'b*tch' and 'b*stard' crop up periodically too.