Staffing is a feature where the player can spend Coin x4000, Staff Ticket x1, or Diamond x40 to randomly draw for one Doctor or Assistant. Diamonds can be spent to add staff only when the player does not have any Staff Tickets left.
In regular staffing, when the player adds staff, there is a 20% chance for a Doctor will appear, and a 80% chance for an Assistant to appear. Normally, these rates are evenly distributed to each character within each category. For example, in a pool with 21 doctors, each doctor has a 0.95% chance to appear. During events, some characters may have their rates increased, decreasing every other character's rates.
It is possible for the player to draw a Doctor or Assistant they already have. If a duplicate is drawn, then that character's skill level will increase instead. Each character can reach a max skill level of 5, after which the character will be removed from the staffing pool. It requires 1, 3, 9, and 16 duplicates, respectively, to reach the next level, which means it requires 29 duplicates in total to bring a doctor/assistant to max level.
Starting from v1.2, the character pools are being limited to 20 Doctors and 40 Assistants. When new characters are added, some characters in the pools will be swapped out. They may still be obtainable from other means, like area clear rewards in Stage Mode or from Battle Boxes.