But television viewers saw another side to the nice-guy Virginia senator as he assumed the role of attack dog against Republican rival Mike Pence.
Kaine, who has described himself as "boring", was picked in July by Hillary Clinton to be her vice-presidential running mate.
Serving in Congress since 2012, he was elected after serving as Virginia's governor for four years - a title he won in part by getting support in areas of the state usually dominated by conservatives.
As governor, Mr Kaine supported expanding early childhood education, digitising historical records, and smoking bans.
While in the Senate, he has served on committees for Armed Services, Aging, Budget and Foreign Relations. He is a former head of the Democratic National Committee.
Before Mr Kaine began his career as a lawyer focused on housing rights, he spent a year volunteering alongside Jesuit missionaries in Honduras. There, he became fluent in Spanish - a skill that could help Mrs Clinton's Hispanic outreach.
He is popular with the White House, and was on the short list for Barack Obama's running mate in 2008. Bill Clinton was said to push for Mr Kaine's appointment to be Clinton's VP.
However, his support for free trade and deregulating local banks make him unpopular with more progressive members of the party.
A practising Catholic, he is personally opposed to abortion. While he is committed to the broader tenets of Roe v Wade, including a woman's right to choose, he supports restrictions on certain types of abortion, parental consent laws and "informed consent" practices.
Although he has lived in Virginia for most of his adult life, and is a huge bluegrass music fan and harmonica player, he is originally from Minnesota and was raised in Kansas. He has a Midwestern appeal that Mrs Clinton could rely on to help gain voters in the Rust Belt.
Virginia is considered a swing state, and Mr Kaine's popularity could improve Clinton's chances there.
Should Mrs Clinton win the election, Virginia's Democratic governor would appoint someone to temporarily fill Mr Kaine's Senate seat, and a special election would be held in 2017.
The reputation that proceeded Tim Kaine before he appeared in the vice-presidential debate was of a mild-mannered Democrat, occasionally known to play his harmonica.