Hikaru no Go/WMG

Hikaru is really messed up, and Sai is his a hallucination or alternate personality.
His go skills are from being a borderline idiot savant, possibly a mild autism combined with OCD that allows him to see the patterns on the board and advance in skill with surprising speed. Sai is a hallucination that he made up based on a book that referenced Shusaku Hon'inbo and other go prodigies (also why nobody ever saw the blood on the board, and how he noticed the signature was a forgery).
 * I believe that Hikaru's Disappeared Dad was a Well Done Son Guy who pounded it into Hikaru's head that boys are supposed to like "real" sports; an "old man's game" like Go would have been completely unacceptable. Sai's personality (deadly serious when playing Go, but Cloudcuckoolander otherwise) is the parts of Hikaru that he's been repressing because he desperately wants to be normal. He's in such deep denial about Sai being part of his personality that he has a Heroic BSOD when Sai "disappears".
 * Well, there is at least one point in the series where Hikaru is told that there is no record whatsoever of a Go Master named Sai, even though information about other Go luminaries from his time period are recorded in detail.

Hikaru and Sai are Calvin and Hobbes.
Short, temperamental kid with crazy hair and his tall, serene imaginary friend who helps with his homework and dispenses philosophy. Early on the imaginary friend points out the the girl in the story is the kid's potential romantic interest (resulting in the kid flipping out). The kid and his imaginary friend frequently play games together. So yeah.
 * I wish to know what sort of crazy game Hikaruball would be. A tag-team cross between Go and Tiddlywinks, perhaps?
 * A 19x19 chalk grid across an athletic field, nine croquet pegs corresponding to the star points, two garbage bags filled with a different color of Frisbee, and no permanent rules.
 * Ah! This is so true! I was very surprised when re-reading Calvin and Hobbes and being constantly reminded of Hikaru and Sai...

Akari is Sayu Yagami.
Based on panels where the two characters appear identical. Hikaru no Go is a universe where most or all of Death Note didn't happen (possibly the chief and Light died early in the series, Witness Protection or marriage accounting for the name change, or, less likely, the universe of the Death Note pilot chapter). Sai is a shinigami who was less interested with being a Chessmaster like Ryuk than he was a literal Go Master, and hid his Death Note in the Go board found at the start of the series. Hikaru could see Sai not because he was destined to be a professional-skill go player, but because he happened to pick up the board first. Hikaru's grandfather, and likely many people since Hon'inbo Shusaku, also were trained in go by Sai, but never showed the same talent and relinquished ownership of the Death Note, retaining only the skills they could remember from having played in tournaments.

Hikaru is the reincarnation of Honinbo Shusaku.
Shusaku regretted not playing Go for himself, so he reincarnated as someone who would never give over their life to Sai. That's why Hikaru's a prodigy, and why he could see Sai. He was already fairly good at Go (letting Sai play through him must have resulted in some rub-off), and he had already known Sai before. That could also explain why Sai's departure hurt him so much-he was losing Sai for the second time.