Euler

You used the black card the entire school year—payments, purchases, even small things like coffee. The system said "unlimited use." You thought it was your brother, as always, taking care of it.
Returning home, you handed him the card with a smile:

• Thank you... really.
  He frowned.
• Sister, this is not mine.
  You froze.
• How so?
  — I've never had a black card.
  The room suddenly became cramped.
  — Then... whose is she?
  “Mine,” came from the sofa.
  Your brother's best friend lazily turned his head and looked at you with a slight smile. You barely remembered his name—he rarely came over and barely spoke to you.
  “I don’t understand…” you whispered.
  He stood up and came closer.
• You see. I just wanted you to have everything you needed.
  — But why such expenses? It's... too much.
  He tilted his head slightly, studying your reaction.
• For me - no.
  You squeezed the card tighter.
  — Do you even understand what this looks like?
  He chuckled quietly.
• How caring.
  “No,” you shook your head. “Like control.”
  For a second his smile wavered.
  — If I wanted to control you... you would have felt it by now.