'You want to calm yourself, Madam Umbridge. You don't want to get yourself into trouble, now.'
--Kingsley Shacklebolt (OP27)
“The Ministry has fallen. Scrimgeour is dead. They are coming.”
--Kingsley Shacklebolt (DH8)
When the Order of the Phoenix reformed in 1995, special effort was made to recruit spies within the Ministry of Magic (OP5). Shacklebolt was one of two Aurors who joined (the other was Tonks). He and Arthur Weasley pretend not to get along with each other at work (OP7).
Although Kingsley was supposed to be the Auror in charge of the search for Sirius Black, his search was a sham. Among other things, he reported that Sirius was in Tibet (OP5); Shacklebolt was so successful at protecting Sirius that the Ministry never found him (OP4-9). Shacklebolt also took turns guarding the entrance to the Hall of Prophecies when it was thought that Voldemort would try to steal the one regarding him and Harry (OP9). Shacklebolt fought with the Order of the Phoenix at the battle of the Department of Mysteries (OP35).
Kingsley's towering size, intellect, power, and calm confidence is frequently reassuring to the others around him. He can be intimidating too: even Dolores Umbridge backed down when he reproached her (OP27). He has a similar effect on Muggles, partly due to his unusual ability to dress like them (DH3). When Scrimgeour became Minister and Shacklebolt was reassigned to undercover security for the Muggle Prime Minister, he quickly became invaluable to the staff at 10 Downing Street (HBP1). Kingsley impressed Vernon Dursley so much that he wanted him to be his escort when the Dursley's evacuated the house on Privet Drive (DH3).
This confidence is never misplaced. It is Shacklebolt who saves Harry and the other guests at Bill and Fleur's wedding when he sends his Patronus to warn everyone that the Ministry had fallen to Voldemort's Death Eaters (DH8).
Shacklebolt became Minister of Magic after Voldemort's defeat (DH36), employing Harry, Ron and Neville (PC/JKR1) immediately after the battle to apprehend the rest of Voldemort's followers. Harry was also asked to help reform the Ministry, and was eventually appointed to be head of the Auror Department (BLC). Together they Harry and Shacklebolt led the Ministry and the wizarding world into much better times.
Patronus: a lynx (DH8).
Skills: Wordless spells (he wordlessly modified Marietta's memory) (OP27), dressing like a Muggle (DH3), calmness under fire, incorruptible reformer.
Name meaning: Shacklebolt="bolt which passes through the eyes of a shackle" (OED). In heraldry, the shacklebolt symbolizes "victory; one who has taken prisoners or rescued prisoners of war." (fleurdelis).