You got the Konrad birthday ending because you worked with Alex the black-haired hotel staff. You can get the other endings by working with the other staff brown-haired and silver-haired. The ending also depends on whether or not you find the secret treasure. This requires you to collect the bottles. 6 endings in total 2 to each hotel front desk staff. the 2 different endings are determined by rather or not you complete the treasure hunt.