In Uruguay,and in my opinion,"una persona seria",means thrustworthy, in whom you can rely,who will not deceive you. Conservative,in Spanish,is conservador. You can check the different meanings of "serio" : serio,ria adj. Severo,poco alegre,grave en el semblante,en actitud y comportamiento: es un niño tranquilo y serio. Enfadado o preocupado: está serio porque le han regañado. Formal y cumplidor: es una persona seria que cumple su palabra. Digno de confianza: es una empresa seria. Que no se destina a hacer reír o divertir,sino a informar,educar,etc.: libros serios; programa de televisión serio. Importante,grave: problema serio. [Ropa,decoración,etc.] de líneas y colores sobrios y poco llamativos: un serio traje de chaqueta. en serio loc. adv. Sin asomo de burla o broma: nos lo dijo muy en serio.

thanks for all the answers, though i'm actually talking about when spanish people are speaking english and they say "I'm a serious person." your responses, though, are helpful for trying to understand the idea they're trying to express.

Persona Seria means trustworthy , reliable, you can be 'una persona seria' and be really funny and liking to tell jokes. Here we can see the difference ,so difficult for non native speakers between 'SER' and 'ESTAR' SER SERIO , to be trustworthy reliable ESTAR SERIO to be serious, sullen stern earnest I hope it helps e.g. Mi padre es una persona muy seria, . Mi padre está muy serio hoy

"Serio" in spanish is someone that is not a good speaker at the begining. Some people use that word for a shy person also