In English, my never changes: my book, my house, my keys. In Portuguese, the word for my changes with the thing you own — its gender, and whether there's one or more:

• o meu livro — my book (masculine)
• a minha casa — my house (feminine)
• os meus amigos — my friends (masculine plural)
• as minhas chaves — my keys (feminine plural)

The owner plays no part. Marcos and Marina both say a minha casa, because casa is feminine — and both say o meu celular (my phone), because celular is masculine.