The Overachiever trait is made for Sims who are always trying to get ahead. It was added with The Sims 4: High School Years and is available to Teen Sims and older.
The biggest advantage of the Overachiever trait is that Sims with it build skills twice as fast. This makes the trait particularly useful if you want a Sim to quickly level up skills for school, university, or their career.
Overachievers also receive the Another For My Repertoire +1 Happy moodlet whenever they level up a skill.
However, the trait comes with some drawbacks. Overachievers can have more difficulty building relationships, so they aren’t necessarily the easiest Sims to socialize with.
Overachievers can also get a Rival +1 Angry moodlet when socializing with another Overachiever.
They can also use the Humble Brag interaction with other Sims.
Teen Overachievers are also more likely to develop certain fears related to failing school or after-school activities.
Overachiever Sims have access to a special Prepare for Next Task interaction.
Using it gives the Sim the Prepared +1 moodlet, which can be Energized, Inspired, or Focused.
This fits the trait pretty well because your Sim can basically prepare themselves for whatever they plan to work on next.
Overachievers don’t like going to work without completing their daily task first.
If they go to work without completing the daily task, they can receive a Tense +2 moodlet.
Overachievers earn career promotions faster.