Additionally to episode 18, Fern and Frieren gets assigned to two different groups (of mages) for the test, so not on the same team. Frieren ends up with two constantly fighting girls and Fern ends up with an imperial court wizard (that killed his opposition to get the position) and a psychopath young girl, that failed her previous graduation from class 3 mage to class 2 mage, because she killed the class 1 mage proctor of the test...
We never really get any hints, that Fern is a curvy young woman besides Frierens references. Probably because she is dressed up like a Taliban bride (minus the headwear), covering her body fully. In Episode 3, Frieren "notices" the differences between the two of them
Stark was a local village hero, because the village thought he subdued the dragon threatening the village (in reality, he was frozen to the spot in fear and the dragon lost interest in him). In episode 6, he gets persuaded by Frieren, that they can kill the dragon, freeing him up, so he can join her and Fern as the groups front line fighter. The plan is, he has to keep the dragon distracted for 30 seconds, giving the mages time to prepare magic for blasting the dragon to pieces. After some very long 30 seconds, Stark has unknowingly defeated the dragon all on his own and gives Frieren an earful because she is holding back and haven't delivered the killing blow yet (because he doesn't know himself, that he actually killed the dragon already, he whinges loudly that he has kept his part of the agreement, but Frieren still hasn't finished the dragon off)