Role in the series
As a child during the Majin Buu arc, Trunks is shown to be quite cocky and stingy, as well as mischievous, most likely due to the fact that this Trunks grew up with his father Vegeta and enjoyed the luxuries of peace, while Future Trunks grew up without Future Vegeta and in an apocalyptic world, causing him to be much more serious and cautious. Trunks has a combination of both of his parents' personalities: cocky and proud like his father and spoiled and selfish like his mother. He refuses to respect someone he does not like, another trait he shares with his father. This is shown when he disrespects Goku when they first meet, as Trunks believes he is not as strong as his father and again when he wondered why he wasn't present when his father was killed. Later on, Trunks does gain more respect for Goku after seeing Goku's Super Saiyan 3 power in action when holding off Majin Buu. He is shown to be loving towards his parents and grandparents and will not think twice about defending his family. Trunks tends to usually take control over the younger Goten, stating to him numerous times how he is both older and stronger (during childhood).
In his later years, Trunks adopts a personality similar to his future counterpart's. During and in between his battles at the World Martial Arts Tournament, he acts just like his father does, calm when he fights and insulting his opponents. He does not act like this however during his battle with Goten because he is his best friend or it could be because it is a battle he could possibly lose. Due to of his maternal side of the family being scientists, he also possesses a significantly huge amount of knowledge about biological and science-related knowledge from a young age, as demonstrated in the movie Dragon Ball Z: Bio-Broly, when he correctly identifies what the Culture Tank equipment is used for at age eight, as well as correctly defining two-thirds of what biotechnology is (the only thing he was not able to answer was what genetic engineering is). Along with inheriting scientific mind of his mother and maternal grandfather, Trunks is also shown to possess a tactical mind like his father and paternal grandfather, as demonstrated by his and Giru deception of Dr. Myuu and his forces on planet M-2 during the Baby Saga.