Kei Hiwatari is Satoshi's adoptive father. In both the anime and the manga, Satoshi and Kei display a mutual forced tolerance of each other in public (his father actually tends to go a bit further and puts on a show of doting and bragging over him), but in realtiy their relationship is tense and sometimes hostile. Although the details of Satoshi's adoption are vague, it is clear the Kei had other intentions when adopting Satoshi (besides just looking for a son), and Satoshi is well aware of the fact that he is nothing but a tool to his father.
Krad is the form Satoshi transforms into due to his Hikari DNA. Krad views Dark and the Niwa family as enemies, and ruthlessly seeks out opporunities to destroy them. Similar to Kei in his regard for Satoshi, Krad sees him as merely a tool. When Satoshi transforms, Krad uses his powers without concern for the damage inflicted on his host's body, and Satoshi ends up in several dangerous situations as a result. Exactly what causes Satoshi's transformation into Krad is vague. While Daisuke tranforms into Dark when confronted with the object of his love and affection, all it seems to take for Satoshi is becoming concerned for or attached to someone else (which would explain his determination to stay away from friends). It is also rumored that wearing his glasses helps to control Krad.
Daisuke Niwa is Satoshi's classmate. Despite Satoshi's stoic and standoffish nature, he takes an interest in Daisuke that eventually developes into a friendship. Without realizing it, Niwa becomes Satoshi's closest (and possibly only) friend, and is one of the few characters that Satoshi displays any kind of concern for. However, due to both boys' condition (the conflict between their families and their other selves, Dark and Krad), it is often difficult for them to maintain their relationship when caught between their duty and their desire to remain friends.
Dark is Daisuke's transformation and the notorious thief that has been stealing the Hikari's artwork for centuries. Satoshi has dedicated his life to capturing Dark, and despite his friendship with Daisuke remains vigilant towards this duty. While Satoshi takes his job very seriously, he enjoys testing Dark's wits, and each failed attempt at capturing the thief is just another opportunity to play with him.
Although there are few instances in both the anime and the manga in which these two characters actually interact, subtle hints suggest a possible future romance between Risa and Satoshi. However, Risa's primary love interest remains to be Dark throughout the bulk of the story, and so a relationship never fully developes between her and Satoshi (up to volume 11 of the manga, at least).