The Republican seemed to stay true to that description on Saturday. He voted in support of repealing the military's "Don't Ask, Don't Tell" policy, but joined a Republican filibuster to block legislation that would have cleared the way to citizenship for children of those in the country illegaly.
Republicans argued, in part, that the The Development, Relief and Education for Alien Minors Act would be too costly for the nation.
Kirk broke with the majority of his party Saturday to repeal a ban on gay people serving in the military, casting one of the first high-profile votes he's faced as Illinois' junior senator.
"As a 21-year Navy Reserve officer, I believe it is important for military leaders, not federal judges, to run our armed forces," he said.
The Senate voted 65-31 to repeal DADT. President Barack Obama was expected to sign the legislation next week, although the change wouldn't take immediate effect. The House had passed an identical version of the bill, 250-175, on Wednesday.
Kirk was one of six Republicans to break ranks. The move was a reversal for him after he'd earlier helped block repeal legislation. A spokesman said as recently as two days ago that Kirk was still undecided on the issue.
On the DREAM Act, Kirk maintained his stance that now was "now is not the time" to enact the legislation.
"The American people believe our borders are broken. It is a fundamental duty of our government to know who is entering the country, making illegal entry nearly impossible. In the coming Congress, we have an overwhelming bipartisan consensus to restore confidence in the security of our borders -- before we pursue other immigration proposals," he said in a statement.
Illinois Sen. Dick Durbin was an original co-sponsor of the bill, which would have allowed illegal immigrants with a high school diploma or a GED to apply for conditional U.S. status if they are under the age of 30 and arrived the U.S. before the age of 16. After a long process -- including two years of service in the military or enrollment in college -- they would then have been eligible to apply for legal immigrant status.