Is it confession that you are my card?

It means that when you are in trouble, even if no one else shares it, at least there is him. I think what he means is that he will still support you even if the whole world abandons you.

Other meaning of the card:

1, that is to say, in poker games, the cards are displayed until the critical moment that determines the outcome. Card is a popular game term in domestic poker and gambling circles.

2. Sentence-making: Only smart people always hold the cards in their hands firmly to prevent problems before they happen, so they always have the last laugh. He saw the opponent's cards clearly, so he won the negotiation.