Sentinels, as defined by Riot, are defensive experts that have abilities that manipulate the battlefield, lock-down areas of contention, and provide utility such as healing.

The main responsibility of a Sentinel player is to NOT LOSE the round. Unlike an initiator or a duelist whose role is to actively win the round by getting on to sites and getting picks. Sentinal and Controllers will actively try not to lose the round by holding sites and flanks.

Sentinels' abilities differ from all the other roles. Often, Sentinel's abilities when placed can last the entire round and can have a lasting impact on the round for its entirety. Think of chamber traps, chyper trips, and killjoy's bots. The only outlier is sage, however, her wall is incredibly powerful by denying an entire entryway, even if it doesn't last the entire round. Due to their lasting presence, Sentinels' abilities have a unique impact on the round and are important in Valorant's current meta.

During defensive rounds, a Sentinel's job is to anchor the site as your team rotates around them. Because of their role, sometimes they are expected to hold a site during a five-man rush. Their job is to either hold it or at least slow the enemy team down until re-enforcements can arrive.

On Offensive rounds, Sentinels have quite a few responsibilities. Holding the flank is the biggest one, in my opinion. If the attacking team dies due to a flanker, that is often a direct failure of the sentinel player. Sentinel's have utility to counter that very play. They need to use their utility or at least have a keen eye on the flank during every attacking round. Secondly, a Sentinel needs to be a trade fragger. They are not expected to be the first or even the second player on-site, however, when those two die after getting contact, a sentinel needs to be on-site to trade those kills. Their job here is to make sure that their team has an equal or more amount of members alive than the enemy. Finally, after the bomb plant, a sentinel needs to hold down the site like they would in defensive rounds.

Overall Sentinel utility is really important. If a round goes by with a Sentinel not using their utility, they have wasted that round and most likely lost that round for their team. Ultimately, everything revolves around the sentinel staying alive as long as possible so their utility has an impact on the game. It is a common mistake that sentinels often get caught dying first. A high Elo sentinel doesn't get caught making that mistake. Dying first although unavoidable sometimes is unacceptable in most circumstances.

Nevertheless, Sentinels also need to be aggressive enough to be unpredictable to the enemy. They can't always be lurking or hiding on-site or only holding flank. Sentinels are in a unique and difficult position because they have a lot of responsibilities. Thankfully, they do have enough utility in order to do all their tasks and hopefully teammates to help them (sadly this is most often optional). Many low Elo players forget to do the ever too crucial task of trade fragging which often leads to them losing countless attack rounds.

To hold a site in a defensive round is not hard. However, to consistently do it over the entire defensive and offensive half (when the spike is planted) requires unpredictability. That attribute is often a result of knowing and using various set-ups and changing holding angles often. Something to keep in mind.

For Sentinel mains, I would recommend learning the kits of all four Sentinels, and based on their kits, the player should come up with their own set-ups during games. This helps players gain a deeper understanding of the strengths and weaknesses of each of the agent's abilities. Furthermore, it will improve how the player adapts to situational changes during the game. Situational changes could include how the enemy reacts to the player's set-ups or their team plays with or against them. Learning a signal "goated" set-up might work in low Elo however, if you don't why or how the enemy is unable to adapt or play around it, it will hinder further progress above silver/gold Elo. Mid to high Elo players love to take advantage of predictable and simple set-ups and positioning.

This is a rough breakdown of the Sentinel class in Valorant. The only thing missing are visual examples of how sentinels differ from other roles and examples of different types of set-ups and what situations they can be effective in.

Also, understand that you could be doing everything correctly as a sentinel player. However, if your movement/aim/game sense is not up to par with the enemy you will lose most of the time. Make sure to work on your overall gameplay and try to have fun. Being a sentinel is not for everyone but I think everyone should try it out at least once in competitive.