Highly Trained

Most SHIELD agents, and even some so-called “super heroes”, don’t have any super-powers. They don’t even have fantastical technology like power armor or mystical artifacts to grant them the ability to do things normal people could not. They are highly trained specialists, who might have a specific combat gimmick (like archery, boomerangs, or dual pistols with specialty ammunition) or use a well-stocked utility belt to respond to the situation, but they’re otherwise a “normal” human being.

There’s many paths a person could walk to achieve the level of training and expertise needed to be at the tier of super-heroes and to be worthy of being recruited into STRIKE. Below are some of the sources, agencies, mentors, and other ways one might achieve that level of training.

SHIELD: The most obvious origin for your extensive training could be simply being a veteran SHIELD agent. SHIELD’s OPS division are some of the best secret agents in the world, and themselves recruit from a variety of different military, law enforcement, and intelligence agencies around the globe. Two of the Avengers are former SHIELD field agents.
MI-13: Much like SHIELD, Britain’s MI-13 is full of elite agents who are recruited from the best of the best among the SAS and other specialist groups. SHIELD has regularly pillaged MI-13’s roster over the years (and MI-13 has done the same in return).
Department H: There’s no current agents on SHIELD’s roster who originate from Canada’s mysterious Department H, but that doesn’t mean there couldn’t be in the future.
ONE: The United States’ Office of National Emergency is a fairly new agency, and was created as a homegrown US response to extranormal threats so the US would not be so reliant on SHIELD. The agents recruited for ONE are fervent patriots, and it would be unlikely that a person recruited for ONE would turn around and join SHIELD months later. But it could happen, especially if they have a falling out with the American agency.
Military Special Forces: Military special forces units have always been a recruiting ground for SHIELD, going back to SHIELD’s origins in the SSR. Britain’s SAS, Canada’s JTF-2, the US’ Seal Team Six or Delta Force, are all examples of special forces units that SHIELD regularly recruits from.
Intelligence and Counter-Terrorism Agencies: A majority of SHIELD’s field agents are experienced members of various national intelligence and counter-terrorism agencies. France’s GIGN, Germany’s GSG 9, Israel’s Mossad, Britain’s MI6, and the myriad of agencies in the US intelligence community form the backbone of SHIELD’s OPS division. There’s an incredibly small number of ex-KGB and FSB agents that are part of SHIELD, due to Russia’s acrimonious relationship with SHIELD.
PMCs: PMCs themselves generally recruit from other military and law enforcement agencies, and groups like Blackguard Inc. recruit almost exclusively from dishonorably discharged military personnel. Nonetheless, it’s possible that someone’s only real combat and field experience comes from former membership in a PMC. These mercenaries are rarely trusted by SHIELD, but sometimes they’re simply too skilled for it to matter where they came from.
Athletes: Some SHIELD field agents have no military, intelligence or law enforcement background. They’re civilian athletes, the best in the world. Former Olympians and champions in their field, prodigies at specific skills that SHIELD simply can’t pass up.
Former Criminals and Terrorists: While SHIELD is reluctant to bring on people with criminal histories, it’s not unheard of. Usually, such individuals possess extranormal powers or expertise of remarkable technology, such as former Leviathan super-soldiers. But from time to time, individuals who show a real drive for redemption and have a trustworthy nature can come from auspicious roots, like ex-HYDRA agents and former bank robbers. These people are under a constant watchful eye from their handlers, and may find that other SHIELD agents resent and/or fear them.
Self-Taught: A small number of SHIELD agents, usually ones that were formerly costumed vigilantes, are so driven to succeed at their chosen goal that they have acquired the skills, training, and knowledge to make them formidable purely of their own study and practice. They may have had many mentors along the way to gain these skills, but ultimately they motivated themselves to be the very best at what they do. Usually, such driven auto-didactic learners are motivated by personal tragedy or a need to right some wrong they see in the world, and may have taken matters into their own hands before SHIELD recruited them.

Suggested Origins: A Death in the Family, The Wake-Up Call, My Legacy, My Mission, What I Know
Suggested Drives: Become an Icon, Die Trying, Do What Others Cannot, Inspire, Keep the Past at Bay, Lead, Protect, Prove Myself, Reconcile My Past, Redeem Myself, See Justice Done

Example Characters: Hawkeye, Black Widow, the Punisher, Whisperer, Mockingbird

Highly Trained

Agents of STRIKE MattZenith MattZenith