Which billionaire co-founded the Giving Pledge with Bill Gates?

Warren Buffett co-founded the Giving Pledge with Bill Gates. The campaign was formally announced in 2010 to encourage wealthy people to commit at least half of their wealth to charitable causes during their lifetimes or after their deaths.

Buffett brought decades of philanthropy and business experience to the initiative. Gates and Buffett recruited other wealthy individuals and families, presenting the pledge as a public commitment intended to make large-scale giving a social norm. Melinda French Gates was also a founding figure and early participant in the campaign.

The Giving Pledge is not a legally enforceable contract. Participants publish letters explaining their intentions, and the pledge generally concerns giving at least 50 percent of wealth rather than donating a fixed amount immediately. Michael Bloomberg, Larry Ellison, and Mark Zuckerberg have all been associated with major philanthropy, but Buffett is the canonical co-founder named alongside Gates in this question.