If a memory or a mind is retentive, it holds facts and does not let them slip away. We use this word about memory, not about boxes or bags — though actually the idea is the same as “keep.” A retentive student remembers names, dates, and new words. It is a fairly formal, slightly old-fashioned word. In everyday talk people just say “She has a good memory.” You almost never call a person “retentive” by itself; you say “a retentive memory” or “a retentive mind.”