In their youth, Wilhelm was angry at Theresia for hiding her identity as the Sword Saint. He took that as her looking down on him and his swordsmanship. However, after finally defeating her, he declared that he would take the sword in her name, so that she wouldn't be forced to fight anymore to protect her loved ones. In their later years, Wilhelm came to deeply love his wife, as he even took her surname van Astrea despite his initial dislike of the Sword Saint's lineage. After her death during the Hakugei expedition, Wilhelm was motivated by his desire for vengeance on the Hakugei.
Wilhelm's relationship with his son is heavily strained, especially since his wife died while on the Hakugei expedition, something that she took on because Heinkel refused to do so.
Wilhelm and Stride held a hostile relationship with each other after the duel of Picoutatte and its Silver Flower Dance. Wilhelm even went as far as to claim that Stride was his mortal enemy after all the bloodshed and tragedy he incurred on the Astrea family and the kingdom of Lugnica.
Wilhelm's relationship with his grandson is very tense, likely because Reinhard inherited his wife's power as the Sword Saint just before she died, making her much weaker than she was at her prime. Nevertheless, Wilhelm did want to fix their relationship after the Hakugei was successfully killed. However, after Reinhard was forced to kill Theresia's resurrected corpse, Wilhelm asked his grandson if he had any regret doing so. After hearing Reinhard's answer, he begins calling the current Sword Saint by his full name, showing that he sees him now as a stranger rather than his grandson.
She was an ally during the times of war, he was commissioned to accompany her in missions with her squad and they united to defeat the Demi Human Alliance. Wilhelm behaved recalcitrantly around her, shooting down her flirtatious nature. However, he deeply cared for her and was devastated after her untimely death.
They accompany Crusch, therefore they know each other very well. Felix knows about his past, even though Wilhelm does not like to talk to people.
Grimm is his former teammate, who had called him to join his group, although all the members were eliminated later. Grimm respects his way of thinking and was amazed by his style of sword fighting. He also follows him in every decision he makes, since he trusts him.
Wilhelm at first saw nothing special in him and didn't like him, but eventually when Subaru helped him beat the Hakugei, he showed respect to him, even protected Emilia and prevented her from being hurt. He also tells him parts of his past, like the wound left by Theresia.