Takato somehow had a dream\vision of Ruki before he met her.
In Episode 7 after Takato tries to tell Ruki about the dream Ruki asked if he has come to confess his love to her, but in a mocking tone. Takto makes a joke about it and she threatens to kick him.
When the Arc is leaving in Episode 41 Takato wants to wait for Ruki almost missing it.
In Episode 42 Takato makes a comment on Ruki's new shirt and both blush.
The phone conversation in the "The Runaway Digimon Express" movie is often cited as evidence.
Rukato is one of the more popular pairings and is seen as the strong girl-timid guy dynamic.