As its name suggests, The Bubbles Card is a mass of soap bubbles. It was created specifically to wash Kero when he is in his true form. It is also used to wash dishes along with other things. It is a generally harmless card.
Bubbles is unusual in that it was only ever referenced in the anime as incidental material, most notably during Episode 36 when its capture was (partially) shown during a replay of Tomoyo's video library. Bubbles' capture is later shown in a bit more detail during the second film, appearing as a large pile of pink bubbles. Bubbles is transformed into a Sakura Card when Sakura uses it to bathe Cerberus in Episode 58.
On its main body, Bubbles appears as a young, mermaid-like little girl wearing a blue gem necklace along with some pearls tied around her bubbly head and turns her blue tail into her blue legs. She is under The Watery.