Editor's note: This story contains graphic descriptions of alleged sexual abuse.
Bellefonte, Pennsylvania (CNN) -- Former Penn State assistant football coach Jerry Sandusky routinely had a then-teenage boy perform oral sex on him while the two showered together on the school's campus and elsewhere, the alleged victim testified Monday.
"It would have to be 40 times, at least," said the now 28-year-old man, adding that the abuse started when he was 14.
Known in court documents as Victim 4, he was the first person to testify at Sandusky's trial in Pennsylvania. The Nittany Lions' longtime defensive coordinator faces 52 charges tied to what prosecutors say was his systemic abuse of at least 10 boys over a span of 15 years. The 68-year-old defendant was under house arrest prior to his trial, which is expected to continue for about three weeks.
Like many alleged victims, the man on the stand Monday met Sandusky through Second Mile, the nonprofit group the ex-coach founded. He described growing up without parental oversight before Sandusky took to him -- playings sports with him; paying for uniforms, a snowboard and other items, taking him to Penn State games and doing other special things. Victim 4 said Sandusky also drove him to buy marijuana once when he was 15 or 16, and had bought him cartons of cigarettes.
Despite what he described as systemic sexual abuse by Sandusky, the witness said he was "scared" and reluctant to tell anyone about it, including peers who admired him for seemingly being well connected with Penn State's football program.
"I didn't want to lose the good things I had," he said.
"I kind of looked at Jerry as a father figure, and he was nice to me except for those other instances ... and I feel cool (because) I'm getting nice things out of it."
Victim 4 said he initially refused to talk to police after reports surfaced Sandusky was being investigated, adding the grand jury was the first to hear his full story.
"I've spent so many years burying this in the back of my head ... but then I found out this happened over and over and over again, forever," he testified. "And I feel if I had just said something back then, this wouldn't have happened to (others). So I feel responsible."
While Sandusky said he wanted him to succeed and was nice to him in public, Victim 4 says their relationship was different in private.
"He treated me like a son in front of other people. Outside of that, he's treating me like his girlfriend," he said, noting Sandusky's habit of putting his hand on the then-teen's thighs when they drove in a car together.
Besides the alleged oral sex, Victim 4 detailed other instances of alleged abuse, including Sandusky trying to penetrate him in the shower, caressing him and "kissing ... my thighs."
This allegedly took place in athletic buildings on Penn State's campus, as well as the Toftrees Golf Resort and hotels -- including on trips to Florida and Texas to watch the Nittany Lions play at the Outback and Alamo Bowl, respectively -- Victim 4 testified.
He described staying in a hotel with Sandusky and his wife on the road to attend one such bowl game, and being in the bathroom getting ready to take a shower when Sandusky walked in.
"He came in and was basically doing the caressing on me, ... and then started to push down on me, to motion me to go down there," the witness testified. "And he said, 'You don't want to go back to Snow Shoe, do you?" referring to the Pennsylvania community where the then-teenager was living.
This incident ended, Victim 4 testified, when Sandusky's wife called out, "Jerry, ... what are you doing in there?" at which point the alleged victim said he "jumped in the shower and locked the door."
Jurors were shown excerpts of letters Victim 4 said Sandusky wrote to him. In one, he writes, "I know that I have made my share of mistakes. ... My wish is that you care and have love in your heart. Love never ends. It bears all things, believes all things, hopes all things."
Defense attorneys had filed a motion earlier Monday seeking to keep out testimony involving prosecutors' allegations Sandusky exhibited "grooming behavior," including the letters to Victim 4.
The lawyers said they intend to offer expert testimony from a psychologist who "will explain that the words, tones, requests and statements made in the letters are consistent with a person who suffers from a Histrionic Personality Disorder," according to documents.
According to the National Institutes of Health, those with histrionic personality disorder "act in a very emotional and dramatic way that draws attention to themselves."
"The goal of a person suffering from this disorder in writing those letters would not necessarily be to groom or sexually consummate a relationship in a criminal manner, but rather to satisfy the needs of a psyche belabored by the needs of such a disorder," the defense lawyers write in their motion.
In opening statements, defense lawyer Joe Amendola suggested his client would take the stand. Sandusky would admit, he said, that he routinely "got showers with kids" after working out.
Sandusky has always maintained his innocence, Amendola said, claiming his client's alleged victims had changed their stories and were questioned until authorities received the answers they wanted.
"A lot of people lied," Amendola said. Some of the alleged victims have civil attorneys, he noted, calling that unusual. Others, he said, have a financial interest in the case.
"One of the keys to this case, one of the keys to your perception ... is to wait until all the evidence is in," Amendola told jurors. "Some of it will be graphic ... it's going to be awful. But that doesn't make it true."
However, he said Mike McQueary, a former graduate student who said he saw what appeared to be Sandusky sexually assaulting a boy in a shower in Penn State's athletic facilities in 2002, did not necessarily lie. McQueary has said he assumed sex was occurring even though he did not see the actual act, Amendola said.
Tom Kline, an attorney for Victim 5, told reporters later that his client had no financial interest and "never sought this out," but considers it "an obligation of citizenship" to testify. And Victim 4 said he's never talked with his lawyer -- whom he said he hasn't paid "a dime" -- about being part of any civil lawsuit against Sandusky.
Amendola told jurors former Second Mile children will testify that Sandusky affected their lives in a positive way, and he later showed letters to Victim 4 in which the ex-coach wrote "I'm proud of you and really care." The defense lawyer also questioned some alleged victims' behavior, like the one who went to a football game with Sandusky prior to his arrest.
Victim 4 brought his girlfriend and baby over to meet Sandusky "like he was bringing his family to meet his father," said the lawyer.
The alleged victim admitted Monday to visiting Sandusky about two years ago, so his girlfriend -- who was suspicious about the past between the two -- "could see that everything was normal."
"But that backfired because the whole time we were there, (Sandusky) only wanted to be with me and was sort of rubbing my shoulders. So (my girlfriend) knew," he testified.
The prosecution presented its opening statements first, during which childhood pictures of eight of the 10 alleged victims were shown on a projector screen. Prosecutor Joseph McGettigan described the extent of each victim's contact with Sandusky.
"You'll hear about systematic behavior by a serial predator. These were experiences that took place not over days, not over weeks, not over months ... but over years," McGettigan said.
Feelings of humiliation, shame and fear led to "years of silence" on the part of accusers, the prosecutor said. He reminded jurors that Sandusky, not Second Mile or Penn State, was on trial. But, McGettigan said, Second Mile represented "the perfect environment for a serial predator."
In interviews after his arrest, Sandusky acknowledged showering and "horsing around" with boys, but denied being sexually attracted to them. McGettigan referred to those interviews during his opening statement, saying, "Deny what you can ... and make an excuse."
A jury of five men and seven women, along with four alternates, was selected last week. Half of the 16 jurors and alternates have ties to Penn State, including one retired professor and one current professor, three graduates, two employees and one current student, showing the prominence of the university in the local community.
The case has raised questions about Penn State's response to allegations, with some claiming the school put its reputation ahead of protecting potential child victims.
University President Graham Spanier and iconic head football coach Joe Paterno lost their jobs soon after Sandusky's arrest amid criticism they didn't handle the matter appropriately. Paterno died of complications from lung cancer in January.
McQueary, the former graduate student who became a coach, is considered a key witness. He said he alerted Paterno in 2002 that he'd seen what appeared to be Sandusky sexually assaulting a boy, an allegation authorities didn't learn of until years later.
Paterno apparently told Athletic Director Tim Curley, but no one notified police. Curley and Gary Schultz, Penn State's senior vice president for finance and business, are now facing felony charges of perjury and failing to report the allegations to authorities. Both of them pleaded not guilty.
On Monday, defense attorneys requested that the grand jury testimony of Curley, Schultz and Spanier be admitted into evidence, saying they anticipate the three would invoke their Fifth Amendment right against self-incrimination if called to testify.
The defense also asked that if prosecutors attempt to enter portions of Sandusky's autobiography, "Touched," into evidence, the entire text be admitted. Defense attorneys said they fear prosecutors may use "select snippets" to mischaracterize Sandusky's "motivations, purposes and actions."
CNN's Laura Dolan and Dana Garrett and In Session's Michael Christian and Jessica Thill contributed to this report.