Błędnie: The partnership between him and the assistant must have been really intimate.
Poprawnie: The partnership between him and the assistant must have been really close.
"Intimate" to 'bliski', 'intymny', 'prywatny'. "Close" to również 'bliski', ale nie musi zawierać odniesienia do sfery prywatności, intymności czy poufałości.