Ryoga Hibiki developed a romantic interest in Akane Tendo when she took his cursed form as a pet, innocently unaware that the pig was truly a human in cursed form, and dubbing him "P-Chan". From that point forward, Ryoga was Ranma's rival for Akane's affection.
Ryoga's feelings may have partially been inspired as a way to get one back at his rival, but they quickly become quite genuine, albeit motivated strongly by Ryoga's loneliness. Ryoga deeply wants to confess his feelings for Akane in person, but is far too shy to muster the will to do it, which is not aided by the fact Ranma readily interrupts Ryoga or plays tricks on him for the fun of it. Ryoga mostly expresses his feelings for Akane with subtle methods; he constantly brings Akane presents and tries to be her best friend, being quick to stand up on her behalf or encourage her. Sometimes, Ryoga's efforts to be Akane's ally lead him to actually being too supportive for her own good, as he is perfectly willing to lie to her or tell her what she wants to hear in order to make it happy. Most notably, he refuses to ever admit that her cooking is bad and will choke it down without complaint; this, in turn, fuels Akane's belief that she is a good cook and Ranma is merely being a jerk when he refuses to eat her food or tells her it tastes bad.
Ryoga's affections for Akane are also the reason why he goes along with the deception of being P-chan, the pet pig. While he does feel somewhat ashamed of it, as he knows it's wrong and that Akane would not be happy to learn she had actually been treating a human being in such a familiar manner - hence why he never confesses his curse to Akane and always panics if it looks like she might be about to figure it out - he is so addicted to the closeness and affection she gives "P-chan" that he readily goes along with it and takes every advantage of it. He is shown at least once to try to avoid seeing her nude, which is admittedly practical given how suspicious a pig with a nosebleed would be, but he still readily goes to bed with her and allows her to cuddle him into her breasts.
By the end of the manga his situation became more complicated when he also develops reciprocated feelings for Akari Unryu. Since meeting Akari, Ryoga has struggled with which girl he wants to pursue - see the Akari-Ryoga Relationship page for further details.
There is no evidence that Akane sees Ryoga as anything more than a friend, despite Ryoga's best efforts to win her romantic affection. She makes a point of acting confused, dismissive or oblivious if someone raises the idea that Ryoga may have a crush on her, and repeatedly refers to Ryoga as a "friend". She evidently seems to think that friendship is enough, and is repeatedly baffled when her calling him a friend seems to make him upset - this is most notable towards the end of the Shishi Hokodan vs Moko Takabisha arc, where she is utterly confused when Ryoga only becomes more depressed when she insists she doesn't hate him and wants them to be "friends forever".
That said, in the ending of the Waterproof Soap arc, Akane comments to the line that after seeing Ryoga's true feelings - and how dangerous he is when emotionally excited - she has no interest in such a dangerous relationship and Ryoga will need to find a girl much stronger than her. Thusly, the possibility exists that after that story, Akane is deliberately feigning ignorance of Ryoga's romantic intentions, in hopes of gently discouraging him from being in love with her, but trying to keep him as a strictly non-romantic friend.