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 head of Berkshire Hathaway is among a group that includes Dan Gilbert, the billionaire founder of Quicken Loans.
US news reports and the Financial Times said Mr Buffett co-signed a letter from the group and pledged money for a bid.
Verizon, private equity firms, and the owner of the Daily Mail have expressed an interest in Yahoo's assets.
Last month the search company, hit by falling advertising and weakening investor confidence, shortlisted about 10 potential buyers.
News of Mr Buffett's interest could improve Yahoo's sale hopes. He admitted at Berkshire's annual meeting last month that his investment firm had been slow to get involved the new tech industry.
Mr Buffett, whose firm has a stake in IBM, said in an interview on 2 May that Yahoo's business had deteriorated significantly and that "something has to change there". But he did not mention any potential bid.
Reuters, which first reported Mr Buffett's involvement, pointed out that Susan Decker, who worked at Yahoo in several senior roles between 2000 and 2009, is now a director on Berkshire's board.
The news agency quoted an interview she gave to CNBC television on 29 April: "I hope the next owner can do something to revitalise the spirit of the core things that made Yahoo very, very unique and create a distinction in consumers' minds about why they love Yahoo still.
"It will be helpful if it is private or part of a much larger corporation to achieve that," she said.
However, Verizon is thought to remain the frontrunner in the bidding for Yahoo.
The US telecoms giant would merge Yahoo with AOL, which it bought for $4.4bn (Â£3bn) last year to create a digital group capable of taking on the likes of Google and Facebook.
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.
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Legendary investor Warren Buffett is backing a consortium bidding for Yahoo's core internet assets, according to reports.