The best character/class a newbie should start on?
It depends what game-style you would like to learn first.
I'll break it down for you, which others may or may not agree with... but how I see it:
Shadowblade: Used for providing support to the team, with powerful poisons and backstab which can be strategically used to kill templars, combats etc. However, Are primarily for stealing relics and capping op's when the enemy is not looking due to their great ability to sneak in meld and shadowstep long distances.
Seeker: Used primarily as a "tank" ... something that can pretty much charge in and cause mass amounts of hand to hand damage. Fury can make the seeker a killing machiene... but it is prone to nerve poison (since seeker attacks are mainly stamina based. However, Some use seekers as trappers and supporters (with bombard)... which can be used to provide huge damage to enemies during combats (with people fighting in the middle and bombard over-head weakening the enemy), and traps protecting relics and getting sneaking sbs etc.
Reaver: The best combat (imo). It can be used as a combat unit to fight along side templars, seekers and other classes. There are a range of powerful attacks, and it is less prone to nerve attacks (due to having mana attacks also). Telekinetic push and hold can aid the team in stopping op cappers and relic thieves etc. Metabolism and sprint make the reaver quick... which can be a useful tool in-game.
Templar: Primary goal is to support the team. They tend to be support (with heal, shield and inspired ressurection)... Which makes fighting a lot easier for the combats (who in turn will protect the templar... hopefully). However, You can also use templars as combats... as they have 3 heavy damage attacks (bash, angelic arc, and blessed strike)... which cause huge amounts of damage to a single or multiple enemies. However, Templar attacks rely on stamina (prone to nerve poison). And heals etc can be interrupted by reavers.
Witch: Another support class. They tend to stay out to the side of fights and use area of effect (AOE) to aid the combats fighting. Things like Cursed earth and Natures wrath can really aid the team. Also, displace and eternal form can aid in protecting or taking ops and/or relics. Witches have the added bonus of having the ability to "gate" ... which is a transportation method for the team and the witch to go anywhere the gate is cast to (with the draw back of nether sickness). However, mana void etc can really help taking out the enemy templar.
Prowler: This is a hard class to explain. The best way to use a prowler IMO is to have scout, shriek, multishot, reinvigorate and entangle. Their quick movement with scout can make relic carrying a lot quicker (thus assisting in winning the game). Their shriek can interrupt anything mana based... and reinvigorate can aid the team a lot (if applied properly) - along with multishot. However, Prowlers can also be used as damage dealers... with aimed shot and prowl - shooting from range and dealing large damage to a single enemy (until another is targetted). Prowl can be useful in protecting templars and other team mates or helping in fights... but a large number of people find them annoying as people tend to run away when the enemy gets close... and then turn and shoot when he gives up the chase.
For new players, I would recommend playing as reaver - as it is the most balanced well skilled class in the game. It is not depended on mana or stamina... but can use both. It is considerably good in combat... but can also offer strategic advantage (with sprint etc).
But this is just my view, and the best thing I can recommend for you is to practice... find a class you like or are good at... and experiment. If you see me in game, I may give you some more "practice" in terms of "hands-on" playing. Ask reverandlamb about my teaching techniques

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