Having characters is essential to doing improv. They can be high energy, low energy, somewhere in the middle - it really doesn't matter as long as they 1)complement your scene partner(s), and 2) don't interfere with the scene, but enhance it.
Everyone goes about creating a character differently. For some, it starts with the body. Perhaps the character has a certain walk. Does Christian Bale as Batman have a different walk than Christian Bale as Bruce Wayne? He should - because they are different characters. Perhaps the hips or stomach sticks out more or the butt pushes backwards. Maybe the hands don't hang perfectly at the side, swing oddly when walking, or have a particular habit when talking. Are the shoulders up by the ears? Does one sag to the side more? How does the face look? Is it pinched? Saggy? Vacant? Intense? These are all the elements that should be thought of when developing a character for the stage.
Some performers start their character with a voice. Sometimes they use an accent or a different dialect. It shouldn't be anything that is too difficult to understand. How will you do a scene with a partner when they are speaking English and no one has any idea what's coming out of your mouth. Unless you're playing Translator, this can thwart a scene quickly. Voices can also be used by simply changing the pitch of your voice to higher or lower. You can change the volume in which you say things. You can add a slight stutter or even add a nasal-sound. Explore the range of your voice and see what you're capable of. Not every character has to have a foreign accent.
Still, some performers are a little more tactile in their development and will create a character around a prop. What happens if you put on glasses? Does your posture and voice change? A certain belt or jacket? What about adding a cane? Sometimes a prop can help you to envision a character in your mind that you can immediately imitate - but be sure that the prop doesn't hinder your stage work.
You can also pick someone that you know or have observed and imitate them. Naturally, your imitation will become almost a caricature of the person you are thinking. Everything seems a little exaggerated but the character is still based on a real person. The odds are high that someone in the audience has met or knows someone like that character. Remember, it's the reality that makes the mockery funny. If the audience can't relate - if they're not in on the joke, so to speak - then they're not going to get it.
At the end of the day, whatever your process for character creation, the important thing is that you continue to develop who this person is. The look and sound of a character is only half the battle. Making them a plausible person is step two. Create a kind of backstory for them in your mind. This way, you are playing an actual character in a scene and not just yourself with an accent and a jacket.