Rangers are known for two things: a wide collection of skills and a mysterious vibe. While some of these skills vary person-to-person, each Ranger must undergo at least 5 years of training before being pronounced a true Ranger, and apprentices are chosen based on certain personality traits.
Consider 'The Red Fox Clan,' for example, where Maddie is assessed on seven skills: shooting (bow and sling), knife throwing, unarmed combat, mapmaking, navigational skills, tactical planning, and unseen movement. Every year, apprentices are given standardized testing, graded as a percent and submitted as a report, to ensure they are on track. Scores in the 90% and above range are considered excellent results. If they fail, their Bronze Oakleaf racks up holes, where too many means they are let go from the program. These third-year assessments are specifically tough, making sure apprentices kept the skills they learned. Other common themes of skills include the double knife sword defense—crucial in 'The Burning Bridge'—and tracking, which is mentioned as an additional testing point in 'A Beast From Another Time.'
The Ranger Corps and Diplomatic Service share the Courier's Code: a system of delivering messages through lanterns. Will is required to memorize and use it for his fourth year assessment as an apprentice, though he does have a decoder as well ('The Siege of Macindaw'). They also share the need to blend in to gather intelligence, pretending they are entertainers, common folk, and so on, to be spoken to and around naturally.
Beyond combat skills, Rangers must have basic wilderness survival knowledge. 'The Battle for Skandia,' mentions that Ranger training involves making snares and cutting a mark in the tree above their placement. 'The Tournament at Gorlan' applauds Halt on his woodcraft, a gift of picking good campsites, and refers to apprentices needing to pass cooking tests, who don't all have a gift for cooking and prefer instead to gather/cut firewood, prepare fires, retrieve water, and clean utensils/pots when in a group.
Rangers also know the fundamentals of medicine, enough to treat wounds on animals (see 'The Sorcerer of the North' and 'A Beast From Another Time'), and to make their own arrows with fletching jigs, as seen in 'A Beast From Another Time' and 'The Hibernian.' The less they depend on an outside party during secretive missions, the better.
A very important part of training are the horses—their care, riding them, and learning the commands. Halt shows off those commands in 'The Kings of Clonmel' to escape, using gestures and noises to instruct Abelard. Additionally, they have a special Ranger pace for traveling (discussed in 'The Tournament at Gorlan'): canter for 20 minutes, lead for 10, and a 10 minute break every 2 hours.
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To be noticed as a teenager and to learn all these skills, Rangers must have several characteristics intrinsic to keeping the Corps competent and credible. They are given a lot of leeway and responsibility, and so must live up to the legend.
A basic truthfulness, even when it sheds oneself in a bad light, is mandatory. If Will had lied about stealing Master Chubb's cakes, he would never have become an apprentice, and he's told that admitting he doesn't know something is better than bluffing ('The Ruins of Gorlan'). Similarly, Gilan is encouraged to acknowledge he's unsure about their progress finding the battle-critical ford in 'The Battle of Hackham Heath.' Rangers should also pay their way, never being beholden to any citizen through guilt-trips ('Duel at Araluen').
A natural curiosity is necessary to notice the little things, especially if they feel off in ways which could spell disaster for a mission. Will is specifically tested for it before his official designation as a Ranger's apprentice, resulting in him breaking into Baron Arald's office in 'The Ruins of Gorlan.'
In the same novel, Will's courage is praised—Rangers must stand their ground no matter the danger, as they are often the last defense between evil and citizens. Faced by a wild boar, Wargals, and Kalkara, Will chose fight rather than flight or freeze.
Creativity and intelligence for problem solving come up time, and time again, within the series. Will's own adventure up north to Macindaw became a practice problem for other apprentices as a good example of stretching thin resources. The Commandant Crowley is noted for his quicksilver brain and tongue, giving him an upper hand on verbal battles with Halt, who in his own right seems to know everything to Will's eyes and has the mental strength to look away from Kalkara. Horace, who is also smart, is intimidated by the smarts of Rangers.
Although Rangers often appear solitary, they can quickly form a well-organized group like in 'The Tournament at Gorlan,' and lead others to carry out the solutions they provide. The entire Corps must move like a well-oiled machine, as there are so few of them to protect the Kingdom.
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Rangers also have their unique skills, often picked up outside their training as a child or for amusement. They set each Ranger apart in good ways, and not so good ways—Halt's well-known forgery skills make an appearance in 'The Battle for Skandia' and 'The Tournament at Gorlan.'
Will is a good climber, starting in his days part of Redmont's Ward. Halt also proved his skills scaling the wall of Castle Wildriver in 'The Tournament at Gorlan,' whereas Crowley outright refused, claiming his skills were inferior.
Most Araluens are comfortable in water, more than Skandians; Hal from the Brotherband Chronicles teaches his friends to swim through learning from his Araluen mother. In the prequel series, Crowley was mentioned to have been seen swimming by Halt, Egon was thrown in the sea by Crowley, and the Rangers sneaking across the river to Morgarath's army were all comfortable in the water. Halt even had to escape his brother's attempt to drown him. In contrast, Will nearly drowned once, as mentioned in 'A New Beginning,' although the river in question may have been too strong for any swimmer to survive.
Will, Crowley, and Berrigan all have musical talent useful for slipping into taverns and becoming friendly with the locals on a quick schedule ('The Sorcerer of the North'). Other Rangers have other, more literal, secret weapons: Maddie's sling, Gilan's sword, and Farrel's battleax.
All of these combined makes Rangers a deadly force. They can go off-grid for weeks at a time, siege a castle, and subtly gather intelligence. What can't they do? Romance, maybe, or child rearing. It's no wonder they seem so magical!