I really wish we saw more about Sora’s relationship with Hikari in their childhood. She should’ve come to see her a lot while playing football with Taichi, as he also played a bit with her sister at some point. They three could come to play together some days and Sora would be just as careful with her as Taichi was since the day Hikari fell ill. Before the incident tho, I easily expect Sora to go easier on Hikari than Taichi would’ve done, so both girls may have shared a cute sister-like relationship.
Beautiful memories like that should not just end right there, so they could still be sharing some activities, like playing football (possibly including Taichi as well) like before or even tennis, which Sora began to practice at some point. Hikari wouldn’t expect to win against Sora but she would still try her best and, above anything else, she’d only desire to spend a good time with a really dear friend, laughing and embracing each other.