SANTA CLARA, Calif. — On college football’s signing day in 2012, Iowa coaches sat around a fax machine and watched it spit out national letters-of-intent. Each piece of paper represented months of diligence and provided its own measure of satisfaction, until one name prompted only sideways glances and confusion.

“We’re all like, ‘Who is George Kittle?’ ” Hawkeyes assistant LeVar Woods said last week. “We hadn’t even been recruiting him. Who is this guy?”

The question — who is that guy? — followed Kittle through the start of his professional career as he knifed through secondaries, bowled over tacklers, cleaved running lanes and generally produced a swath of mayhem in his wake. Nobody is asking anymore. Kittle has become one of the best tight ends and most destructive forces in the NFL, a grinning, maniacal face of the San Francisco 49ers’ march to the NFC championship game, which they will host Sunday at 6:40 p.m. against the Green Bay Packers.

Kittle caught 85 passes for 1,053 yards and five touchdowns in 14 games, and receiving may be only the second-best part of his game. Kittle is a devastating run blocker, equal parts dominant and willing. He grades his performance solely on how he blocks. Most tight ends ask for the ball. Kittle sometimes tells Coach Kyle Shanahan which player on the opposing defense he wants to crush, and Shanahan finds a play-call to accommodate him.

“The run plays he suggests are kind of half-thoughts,” Shanahan said. “They’re more to, how he can hit someone?”

Kittle hurt his ankle in last weekend’s victory over the Minnesota Vikings. He stood on the sideline at Wednesday’s practice in sweats and a knit cap. He participated in full Thursday and left no doubt he would play.

“I feel fabulous,” he said. “Thanks for asking.”

The 49ers are playing for a trip to the Super Bowl, but those who know him would have expected the same response if Sunday’s game was two-hand touch in the backyard. Kittle plays with both brutality and bliss. On one play this season, he laughed while pile-driving an Atlanta Falcons safety onto his back in the end zone. On a game-sealing catch, a New Orleans Saints defender grabbed his face mask and Kittle dragged him for 15 yards. Inside his left forearm, he has a tattoo of Heath Ledger’s Joker.

“The mentality of being an ass-kicker,” 49ers offensive tackle Mike McGlinchey said. “That mentality has never changed for him.”

Kittle reached stardom from an unlikely launching point. He was a lanky, 185-pound wide receiver and safety at Norman High in Oklahoma who, in his words, “didn’t get recruited.” He always believed, from the time he was a little boy, he would be an NFL player. That belief saved him at a point when he could have squandered his career. It led him to ask himself: Who is George Kittle?

A look in the mirror

Kittle’s father, Bruce, played on the offensive line at Iowa and became a football coach. He moved the family to Norman after he got a job on Oklahoma’s staff. Most of Kittle’s recruiting attention came from service academies — he was a near-4.0 student, high school coach Greg Nation said — and Football Championship Subdivision schools.

Kittle always dreamed about playing for the Hawkeyes. Oklahoma Coach Bob Stoops called Iowa Coach Kirk Ferentz, who had coached Bruce Kittle as an Iowa assistant, and recommended he consider Kittle. Kittle would have walked on to play at his father’s alma mater. When a last-minute scholarship came available, Ferentz offered it to Kittle.

The Iowa coaches who never tracked Kittle in high school quickly realized they had something in their mysterious recruit; they just didn’t know what. They considered playing him on both offense and defense. Was he a wide receiver? A linebacker? Could he add enough weight to play defensive end? They just knew he could run like hell and he approached football with the pure zeal of a dog chasing a stick.

Coaches landed on making him a tight end, and his speed stood out. He torched defenders while playing on the scout team with quarterback C.J. Beathard, whom the 49ers would eventually draft in the same class as Kittle. When Kittle covered kicks in practice, nobody could stop him.

Despite his raw talent, Kittle redshirted as a freshman and barely saw the field his first two seasons in uniform. His goal remained to make the NFL, but he recognized he had veered off track. He partied too much, drank too much and hung out with friends when he should have been studying football. He had to ask himself a basic question: Who did he want to be?

“You just got to look in the mirror and see what the problem is,” Kittle said. “I just looked in the mirror and said: ‘This isn’t what you want. It’s your fault. So fix it.’ ”

One moment served as a hinge. Pat Angerer, a former Iowa linebacker who played for the Indianapolis Colts, returned to campus as a coaching intern after Kittle’s sophomore season. Ferentz told the team Angerer barely played for three years but changed his behavior and became a second-round NFL draft pick.

“I asked him, ‘What did you do?’ ” Kittle said. “He said: ‘I stopped drinking so much. I stopped going out so much. I stopped fighting people when I got drunk and focused on football.’ I said, ‘Well, I’ll do the same thing.’ I didn’t really fight anybody, but I changed all that up. And I am where I am.”

Kittle had caught six passes in his career. But he had packed on about 40 pounds of muscle since high school without losing any speed. At one spring practice, Kittle squared off against a senior leader who had been all-Big Ten the prior season.

“George came off the ball and put this guy on his back,” said Woods, who became Iowa’s tight ends coach that season. “Everyone started paying attention at that point. Everyone was like, ‘This is for real.’ ”

Kittle took off. He flashed his speed as a receiver, averaging 14.4 yards per catch in his two seasons as a starter, but Iowa mostly used him as a blocker in its run-heavy offense. He came to love blocking, honing his technique but mostly developing a passion for it. Iowa practices included endless sessions of nine-on-seven run-blocking drills.

“If you don’t enjoy it, you’re going to be miserable,” Kittle said. “So I just enjoyed it."

During games, Kittle would celebrate blocks more than catches. “When he’d make a big block, he’d get up and he’d start rolling his arm like he’s shooting dice or something like that,” Woods said, laughing. Teammates voted to make him a captain. In one game early in his senior season, against North Dakota State, coaches marveled at his ferocity.

“There were times when George was taking a guy over the bench,” Woods said. “There’s a couple pictures where if you were to watch that tape, you’re like: ‘Man, what the heck? He is trying to kill this guy.’ ”

Always moving forward

Kittle finished his college career with 10 touchdowns but only 48 catches. The 49ers studied his traits more than his stats. His combine numbers were elite, and his film showed a powerful blocker perfectly suited to Shanahan’s scheme. The 49ers graded Kittle as one of the best tight ends in the draft.

Because they believed other teams were overlooking him based on his college production, they estimated he would be picked in the third round. The way their draft broke, other needs superseded tight end. They took cornerback Ahkello Witherspoon and Beathard, Kittle’s college teammate, in the third round and nabbed running back Joe Williams in the fourth.

By the time the 146th overall pick arrived, eight tight ends had been chosen, including Jake Butt by the Denver Broncos with the 145th.

“We were just shocked he was still sitting there,” Shanahan said.

Kittle became the 49ers’ starting tight end as a rookie. Last season, he gained 1,377 yards on 88 catches and was second-team all-pro. This season, he made the first team and became one of the faces of San Francisco’s youthful, rambunctious powerhouse. Outside Levi’s Stadium, one banner shows Kittle squeezing a football and trying to scowl but really smiling.

In Woods’s office at Iowa, he keeps a picture of the time Kittle bear-hugged him so hard on the sideline at the end of a victory that he lifted the coach off the ground. Woods thought he had broken a rib. Kittle then did the same thing to Woods’s son, who was 8 at the time. “You can see the smile on my son’s face and on George’s face,” Woods said.

Kittle stills plays as if he savors every second on the field, every chance to be an ass-kicker. Even at his lowest point, when nobody else saw him as an NFL player, Kittle did.

“If I wouldn’t have felt that,” Kittle said, “I probably wouldn’t have changed anything.”

Kittle recognized what held him back, made the necessary changes and became one of the best in the world at what he does. He loves the least glamorous part of his job the most. When things are hard, he is at his best. That is who George Kittle is.