"Ursula: It has been rumoured that if you obtain a medic level of over 80
with the second number being odd (eg: 83, 87, 93) you will obtain Ursula
assuming you don´t already have Elisa on your team. I have tried this
by discharging Elisa and beating the game with both a medic level of 83
and 99 and I did not unlock Ursula, but rather got Elisa back. So I think
it is safe to say this is not the case. I have yet to try all odd
numbers, but I think this is simply not the case."
Source :
http://www.gamefaqs.com/pportable/psp/file/932978/46364
Apparement Ursula serait aléatoire, tu as autant de chance d´avoir Elisa que Ursula, mais tu ne peux pas avoir les deux !