The Sharingan

The Sharingan grows from one tomoe to three, each stage unlocking more of its power.

Tomoe Growth

A newly awakened Sharingan has 1 tomoe.
The 2nd and 3rd tomoe wake near death, like a bloodline, but only once your maximum chakra pool is big enough: 4,000 for the 2nd tomoe and 9,000 for the 3rd.
Trigger: after damage lands from a player or creature (not self-inflicted, not environmental), if you have learned the Sharingan, haven't reached 3 tomoe yet, meet the chakra threshold, and are at or below 25% of your max health.
When all of that is true, the next tomoe may grow. It comes more readily than a bloodline awakening.
Cooldown: once it has had its chance (grown or not), it can't try again for 10 minutes (real time). This cooldown survives death.
On growth: if the plain Sharingan is open it updates on the spot, spins into the new tomoe and flares. A red title, "The Second Tomoe" or "The Third Tomoe", appears with the subtitle "Your Sharingan matures", plus a chat message ("On the edge of death, your eyes see more: a second tomoe has grown in your Sharingan." or "...your Sharingan has matured to three tomoe."), 40 red glow particles at the eyes and a heartbeat sound.
An implanted Sharingan (or Mangekyo) in either socket counts as all three tomoe at once.

1 tomoe: the lock-on (15 seconds).
2 tomoe: nothing extra of its own. Copying is the learned technique Sharingan: Copy & Store Jutsu, usable at any tomoe.
3 tomoe (and every Mangekyo): the lock-on lasts 60 seconds.

Needs a Sharingan (any tomoe) or a Mangekyo open.
Locks onto the entity you aim at within 40 blocks, or failing that the first one in a 30-degree cone in front of you.
Lasts 15 seconds (1-2 tomoe) or 60 seconds (3 tomoe or Mangekyo). Press K again to release.
While locked, your view eases round toward the target, keeping it centred.
Every jutsu that looks for "the aimed target or the nearest" picks the locked target first if it's within that jutsu's range, so your jutsu home in on it.
The lock ends when time is up, the target dies, it gets farther than about 49 blocks away, or the Sharingan closes.
Message: "Locked on: <name>".

Copying is a technique you learn in the Sharingan tree: Sharingan: Copy & Store Jutsu (rank B, 300 chakra, 20-second cooldown, no hand signs), cast from your jutsu bar with the Sharingan open. There is no separate copy key.
It locks on to your target if you aren't locked on already ("Lock on to a shinobi to copy their jutsu." if there is no one), then reads them for 10 seconds ("Reading their jutsu... (10 s)"). The first nature-release jutsu (one that belongs to a nature such as fire, water or lightning) that the locked-on shinobi casts in that window is read. Players and NPC shinobi can both be copied.
The copy ends when a jutsu is read, the 10 seconds are up or the lock-on is lost. Whatever was read is kept: it becomes your one stored copied jutsu, replacing any earlier one ("Copied: <jutsu>", with the Sharingan sound and a red glow). If nothing was read: "There's no technique to copy".
I casts the stored copied jutsu with its normal hand signs, for twice its chakra cost, followed by a 60-second cooldown counted from when it releases. It is cast even though you never learned it.

With any Sharingan (any tomoe) or any Mangekyo open, every attack from a creature or player has a 25% chance to be read and slipped: melee blows, their jutsu, their projectiles and their summons' attacks alike.
A slipped attack is cancelled completely. In a burst of wind and smoke you reappear just behind whoever made it, facing their back. If there's no room behind them, you sidestep instead.
At most one attack a second can be slipped.

All of these need the Sharingan (or a Mangekyo) open; this is checked before any chakra is spent.

Genjutsu: Sharingan

Rank: C. Cost: 280 chakra. Cooldown: 6.0 seconds. Hand signs: 2.
Needs a target within 24 blocks.
Paralyses the target for 5 seconds (2 seconds on players), plus Nausea for 8 seconds and Darkness for 5 seconds.
A red ring ripples at the target's eyes.

Sharingan: Foresight

Rank: B. Cost: 780 chakra. Cooldown: 30 seconds. Hand signs: 4.
For 8 seconds, every hostile hit that comes from an attacking entity is cancelled, with wind particles.
Also grants the Shadowless effect for 8 seconds (+220% movement speed, +150% attack speed, +5 attack damage).

Copy Wheel Eye: Mirror Jutsu

Rank: A. Cost: 2,500 chakra. Cooldown: 22.5 seconds. Hand signs: 5.
Targets the player you aim at (or the nearest) within 30 blocks.
If that rival cast a jutsu within the last 20 seconds, you release that same technique at once, aimed by you and mirrored straight back at them, at no further cost.
Nothing is kept: to keep a jutsu and cast it later, use Sharingan: Copy & Store Jutsu.
It can't copy Susanoo or Mangekyo-opening jutsu. It only works on players; otherwise "nothing copied".

Izanagi

Rank: S. Cost: 6,100 chakra. Cooldown: 48 seconds. Hand signs: 7.
Arms Izanagi for 12 seconds and remembers where you stood.
If a blow in that window would kill you (damage at least equal to your health plus absorption), the damage is cancelled, you are healed to full, teleported back to where you cast it, every effect is cleared, your eyes close, and you are blinded for 10 seconds.
A totem sound and a red burst, with the message "Izanagi: reality rewritten. Your eye goes dark."
One use per arming. The eye isn't lost: "goes dark" means your eyes close and you are blinded for 10 seconds.