In order to obtain other digimon within Digimon Battle City you must complete an S-Rank mission with a digimon as a prize. S-Rank mission objectives are never exactly the same as another. One may be dice rolling, another may be a battle. An example of this mission would be the one below.

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Mission!
Objective: Battle Tentomon!
Description: Feisty Tentomon will pick fights and never show up when he sees a larger digimon. Unfortunately his reputation has plummeted. Can you reignite Tentomon's will to fight?
Requirements: You must battle and defeat Tentomon in a 1v1 battle.
May only attempt once per day.
Rank:S
Reward: Tentomon as an extra partner

For a S-Rank battle you are only allowed one battle per day. In a S-Rank battle the opposing digimon will have 100 stats above you in each category along with a special feature based on the mission. For a S-Rank dice roll mission you are only allowed three attempts per day.